vp_vdpa: report doorbell address

This patch reports the per vq doorbell location and size to vDPA
bus. Userspace can then map the doorbell via mmap() via vhost-vDPA bus
driver.

Signed-off-by: Jason Wang <jasowang@redhat.com>
Link: https://lore.kernel.org/r/20210415073147.19331-8-jasowang@redhat.com


Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Reviewed-by: Eli Cohen <elic@nvidia.com>