virt: allow read Hostname from libvirt metadata

virt: allow read Hostname from libvirt metadata

This change allows to read Hostname from Libvirt metadata API.

Applications like OVirt or Openstack Nova uses this metadata API to
store additional informations like the 'user facing' Guest name.

To do so, a new choice 'metadata' is added for 'HostnameFormat' and
'PluginInstanceFormat'. And two new options are also added to localize
the hostname into the Guest metadata: HostnameMetadataNS and
HostnameMetadataXPath.

Closes: #2805