fix qemu_use_agent to work with windows guests (#1599)

When using `qemu_use_agent = true`, the provisioning of a Windows
machine was faling to get the IP address with the error:

```
`get_ipaddress_from_qemu_agent': undefined method `downcase' for nil:NilClass (NoMethodError)
```

```plain
Traceback (most recent call last):
	18: from /opt/vagrant/embedded/gems/2.3.0/gems/logging-2.3.1/lib/logging/diagnostic_context.rb:474:in `block in create_with_logging_context'
	17: from /opt/vagrant/embedded/gems/2.3.0/gems/vagrant-2.3.0/lib/vagrant/action/builtin/wait_for_communicator.rb:16:in `block in call'
	16: from /opt/vagrant/embedded/gems/2.3.0/gems/vagrant-2.3.0/plugins/communicators/winrm/communicator.rb:31:in `wait_for_ready'
	15: from /opt/vagrant/embedded/lib/ruby/2.7.0/timeout.rb:110:in `timeout'
	14: from /opt/vagrant/embedded/lib/ruby/2.7.0/timeout.rb:33:in `catch'
	13: from /opt/vagrant/embedded/lib/ruby/2.7.0/timeout.rb:33:in `catch'
	12: from /opt/vagrant/embedded/lib/ruby/2.7.0/timeout.rb:33:in `block in catch'
	11: from /opt/vagrant/embedded/lib/ruby/2.7.0/timeout.rb:95:in `block in timeout'
	10: from /opt/vagrant/embedded/gems/2.3.0/gems/vagrant-2.3.0/plugins/communicators/winrm/communicator.rb:37:in `block in wait_for_ready'
	 9: from /opt/vagrant/embedded/gems/2.3.0/gems/vagrant-2.3.0/plugins/communicators/winrm/helper.rb:21:in `winrm_info'
	 8: from /opt/vagrant/embedded/gems/2.3.0/gems/vagrant-2.3.0/plugins/communicators/winrm/helper.rb:35:in `winrm_address'
	 7: from /opt/vagrant/embedded/gems/2.3.0/gems/vagrant-2.3.0/lib/vagrant/machine.rb:452:in `ssh_info'
	 6: from /home/rgl/.vagrant.d/gems/2.7.6/gems/vagrant-libvirt-0.10.7/lib/vagrant-libvirt/provider.rb:58:in `ssh_info'
	 5: from /home/rgl/.vagrant.d/gems/2.7.6/gems/vagrant-libvirt-0.10.7/lib/vagrant-libvirt/provider.rb:98:in `state'
	 4: from /home/rgl/.vagrant.d/gems/2.7.6/gems/vagrant-libvirt-0.10.7/lib/vagrant-libvirt/driver.rb:194:in `state'
	 3: from /home/rgl/.vagrant.d/gems/2.7.6/gems/vagrant-libvirt-0.10.7/lib/vagrant-libvirt/driver.rb:103:in `get_domain_ipaddress'
	 2: from /home/rgl/.vagrant.d/gems/2.7.6/gems/vagrant-libvirt-0.10.7/lib/vagrant-libvirt/driver.rb:242:in `get_ipaddress_from_qemu_agent'
	 1: from /home/rgl/.vagrant.d/gems/2.7.6/gems/vagrant-libvirt-0.10.7/lib/vagrant-libvirt/driver.rb:242:in `each'
/home/rgl/.vagrant.d/gems/2.7.6/gems/vagrant-libvirt-0.10.7/lib/vagrant-libvirt/driver.rb:243:in `block in get_ipaddress_from_qemu_agent': undefined method `downcase' for nil:NilClass (NoMethodError)
```

This is due to Windows qemu-ga reporting interfaces that do not have a
`hardware-address`. For example, the `Loopback Pseudo-Interface 1` does
not have it:

```console
$ virsh qemu-agent-command openwrt-vagrant-windows '{"execute":"guest-network-get-interfaces"}' --pretty
{
  "return": [
    {
      "name": "Ethernet 2",
      "ip-addresses": [
        {
          "ip-address-type": "ipv4",
          "ip-address": "10.0.20.11",
          "prefix": 24
        }
      ],
      "statistics": {
        "tx-packets": 3090,
        "tx-errs": 0,
        "rx-bytes": 50713786,
        "rx-dropped": 0,
        "rx-packets": 35116,
        "rx-errs": 0,
        "tx-bytes": 744040,
        "tx-dropped": 0
      },
      "hardware-address": "08:00:27:00:00:0b"
    },
    {
      "name": "Loopback Pseudo-Interface 1",
      "ip-addresses": [
        {
          "ip-address-type": "ipv6",
          "ip-address": "::1",
          "prefix": 128
        },
        {
          "ip-address-type": "ipv4",
          "ip-address": "127.0.0.1",
          "prefix": 8
        }
      ],
      "statistics": {
        "tx-packets": 0,
        "tx-errs": 0,
        "rx-bytes": 0,
        "rx-dropped": 0,
        "rx-packets": 0,
        "rx-errs": 0,
        "tx-bytes": 0,
        "tx-dropped": 0
      }
    }
  ]
}
```

With this change, the IP address can now be found without errors:

```plain
...
    windows: WinRM address: 10.0.20.11:5985
    windows: WinRM username: vagrant
...
```

lib/vagrant-libvirt/driver.rb

@@ -239,6 +239,7 @@ module VagrantPlugins
 
         unless addresses.nil?
           addresses['return'].each do |interface|
+            next unless interface.key?('hardware-address')
             next unless domain.mac.downcase == interface['hardware-address'].downcase
 
             @logger.debug("Found matching interface: [#{interface['name']}]")