engine: fetch exit reason from VDSM

VDSM has gained the capability to report the
reason why a VM is gone in `down` status.
see http://gerrit.ovirt.org/#/c/22631/

This patch adds support for this field;
backward compatibility with old VDSM lacking
this field is ensured.

Feature page: http://www.ovirt.org/Features/Detailed_Exit_Reason

Relates-To: https://bugzilla.redhat.com/557125
Relates-To: https://bugzilla.redhat.com/697277
Change-Id: Ib36f06f3ba63a743d624e417fe2c96b84dda60be
Signed-off-by: Francesco Romani <fromani@redhat.com>