virtio: right size for virtio_queue_get_avail_size

Being working on dataplane I notice something strange:

virtio_queue_get_avail_size() used a 64bit size index
for the calculation of the available ring size.

It is quite strange but it did work with the old calculation
of the avail ring, at most with performance penalty,
and I wonder where I missed something.

This patch let use a 16bit size as defined in virtio_ring.h

Signed-off-by: Pierre Morel <pmorel@linux.vnet.ibm.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>