vhost: replace device ID in vDPA ops

This patch is a preliminary step to get rid of the
vDPA device ID. It makes vDPA callbacks to use the
vDPA device struct as a reference instead of the ID.

Signed-off-by: Maxime Coquelin <maxime.coquelin@redhat.com>
Acked-by: Adrián Moreno <amorenoz@redhat.com>