vdpa: fix comment of vdpa_register_device()

The function should be called only after a success calling for
vdpa_alloc_device().

Reported-by: Eli Cohen <eli@mellanox.com>
Signed-off-by: Jason Wang <jasowang@redhat.com>
Link: https://lore.kernel.org/r/20200413093738.28467-1-jasowang@redhat.com


Signed-off-by: Michael S. Tsirkin <mst@redhat.com>