vhost: allow per device message handler

This patch allow device to register its own message handler during
vhost_dev_init(). vDPA device will use it to implement its own DMA
mapping logic.

Signed-off-by: Jason Wang <jasowang@redhat.com>
Link: https://lore.kernel.org/r/20200326140125.19794-3-jasowang@redhat.com


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