virtio: push out code to vp_avq_index()

To prepare for the follow-up patch to use the code as an op, push out
the code that gets admin virtqueue base index and count to a separate
helper.

Signed-off-by: Jiri Pirko <jiri@nvidia.com>
Message-Id: <20240716113552.80599-8-jiri@resnulli.us>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>