vm: graphdev: fix device hooks post migration

the _deviceXML attribute was not regenerated on
destination VM after migrations.

Change-Id: I6643f81cc5cff1a282da607e7e5759a97b081c7a
Signed-off-by: Francesco Romani <fromani@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/27933
Reviewed-by: Dan Kenigsberg <danken@redhat.com>