Bugfix update device hooks: after_update_device failure.

In case of failure during before update device, after_update_device
should use the backup object, having vnicXML not in a valid state.

Change-Id: I0ab64f88e697e48ad2a3552d1361742846871ed8
Bug-Url: https://bugzilla.redhat.com/show_bug.cgi?id=893576#c11
Signed-off-by: Giuseppe Vallarelli <gvallare@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/15982
Reviewed-by: Dan Kenigsberg <danken@redhat.com>
Tested-by: Dan Kenigsberg <danken@redhat.com>