vm: report exit reason for migration failed

this patch add reporting for an explicit exit
reason when migration fails, instead of
generic error.

Change-Id: Iab6cb810138e751f8fde784c5f6e0fdaf0f51fa2
Relates-To: https://bugzilla.redhat.com/697277
Signed-off-by: Francesco Romani <fromani@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/25164
Reviewed-by: Dan Kenigsberg <danken@redhat.com>