packaging: Depend on the qemu-guest-agent

For support of freezing and thawing the filesystem for the
Backup and Restore API feature of oVirt

Change-Id: I4c550801326dfa0b8d9357770f48a6d9b64eb3a1
Bug-Url: https://bugzilla.redhat.com/1055524
Signed-off-by: Vinzenz Feenstra <vfeenstr@redhat.com>