vdpa: remove hard coded virtq num

This will enable vdpa providers to add support for multi queue feature
and publish it to upper layers (vhost and virtio).

Signed-off-by: Max Gurtovoy <maxg@mellanox.com>
Reviewed-by: Jason Wang <jasowang@redhat.com>
Link: https://lore.kernel.org/r/20200804162048.22587-7-eli@mellanox.com


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