Inject default qemu_agent allowing for explicit disable (#1408)

Facilitate the injection of a default channel when qemu_agent is set to
true to make enabling it's use a simple `qemu_agent = true` for most
cases.

Where custom values are needed, they can be provided and the code will
skip adding a corresponding entry, or if necessary the user can add an
entry and mark it disabled similar to how synced folders work.

Closes: #1341

tests/qemu_agent/Vagrantfile

@@ -7,7 +7,6 @@ Vagrant.configure("2") do |config|
   config.vm.box = "generic/debian10"
   config.vm.synced_folder ".", "/vagrant", disabled: true
   config.vm.provider :libvirt do |libvirt|
-    libvirt.channel :type => 'unix', :target_name => 'org.qemu.guest_agent.0', :target_type => 'virtio'
     libvirt.qemu_use_agent = true
   end
 end