vmgenid: use UUID property type

vmgenid: use UUID property type

Switch vmgenid device to use the UUID property type introduced in the
previous patch for its 'guid' property.

One semantic change it introduces is that post-realize modification of
'guid' via HMP or QMP will now be rejected with an error; however,
according to docs/specs/vmgenid.txt this is actually desirable.

Signed-off-by: Roman Kagan <rkagan@virtuozzo.com>
Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Reviewed-by: Ben Warren <ben@skyportsystems.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>