vdpa_sim: set vringh notify callback

Instead of calling the vq callback directly, we can leverage the
vringh_notify() function, adding vdpasim_vq_notify() and setting it
in the vringh notify callback.

Suggested-by: Jason Wang <jasowang@redhat.com>
Acked-by: Jason Wang <jasowang@redhat.com>
Signed-off-by: Stefano Garzarella <sgarzare@redhat.com>
Link: https://lore.kernel.org/r/20201215144256.155342-15-sgarzare@redhat.com


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