vdsm: Make guest agent FQDN information available

The FQDN value is the fully qualified domain name of the guest OS.
This might be on not properly configured systems only 'localhost.localdomain',
however for properly configured systems this is a very useful information.

Change-Id: Ib6d92bbfc7c4d3004f12376bc44dafae1e339cdd
Signed-off-by: Vinzenz Feenstra <vfeenstr@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/16572
Reviewed-by: Barak Azulay <bazulay@redhat.com>
Reviewed-by: Dan Kenigsberg <danken@redhat.com>