vdpa: Avoid compiler to squash reads to used idx

vdpa: Avoid compiler to squash reads to used idx

In the next patch we will allow busypolling of this value. The compiler
have a running path where shadow_used_idx, last_used_idx, and vring used
idx are not modified within the same thread busypolling.

This was not an issue before since we always cleared device event
notifier before checking it, and that could act as memory barrier.
However, the busypoll needs something similar to kernel READ_ONCE.

Let's add it here, sepparated from the polling.

Signed-off-by: Eugenio Pérez <eperezma@redhat.com>
Signed-off-by: Jason Wang <jasowang@redhat.com>