Handle VM not accessible during reboot (#1367)

To support commands requesting a reboot of a VM after execution, the
query of ssh_info needs to avoid triggering an error when the IP address
is not yet retrievable as this indicates the VM would not be reachable.

Wrap the returning of the state in the driver to distinguish between the
following states:
- :running - indicates the machine is available
- :inaccessible - the machine is running but not yet available to
  connect

This is based on the behaviour from the virtualbox provider.

Includes some rudimentary tests to exercise the driver state code.

Closes: #1366

spec/unit/action/destroy_domain_spec.rb

@@ -12,14 +12,18 @@ describe VagrantPlugins::ProviderLibvirt::Action::DestroyDomain do
   include_context 'unit'
   include_context 'libvirt'
 
+  let(:driver) { double('driver') }
   let(:libvirt_domain) { double('libvirt_domain') }
   let(:libvirt_client) { double('libvirt_client') }
   let(:servers) { double('servers') }
 
+  before do
+    allow(machine.provider).to receive('driver').and_return(driver)
+    allow(driver).to receive(:connection).and_return(connection)
+  end
+
   describe '#call' do
     before do
-      allow_any_instance_of(VagrantPlugins::ProviderLibvirt::Driver)
-        .to receive(:connection).and_return(connection)
       allow(connection).to receive(:client).and_return(libvirt_client)
       allow(libvirt_client).to receive(:lookup_domain_by_uuid)
         .and_return(libvirt_domain)