Handle and store serial to guest device names mapping

The serial to guest device name mapping helps to identify 'physical' hard
drives in the guest OS from the outside and correlate our disk images to
those drives and their name in the guest OS.

Guest agent implementation: http://gerrit.ovirt.org/#/c/31465/

Change-Id: I69db18414b17b23eb7bcbfbd8b2584f622e53276
Bug-Url: https://bugzilla.redhat.com/1127607
Signed-off-by: Vinzenz Feenstra <vfeenstr@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/31497
Reviewed-by: Allon Mureinik <amureini@redhat.com>
Reviewed-by: Liron Aravot <laravot@redhat.com>
Reviewed-by: Francesco Romani <fromani@redhat.com>
Reviewed-by: Dan Kenigsberg <danken@redhat.com>