Only qcow2 driver supports vmstate.
In qcow2 these requests go through .bdrv_co_p{read,write}v_part
handlers.

So, let's do our basic check for the request on vmstate generic
handlers.

Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Message-Id: <20210903102807.27127-2-vsementsov@virtuozzo.com>
Signed-off-by: Eric Blake <eblake@redhat.com>