vdpa: set the virtqueue num during register

This patch delay the queue number setting to vDPA device
registering. This allows us to probe the virtqueue numbers between
device allocation and registering.

Reviewed-by: Stefano Garzarella <sgarzare@redhat.com>
Signed-off-by: Jason Wang <jasowang@redhat.com>
Link: https://lore.kernel.org/r/20210223061905.422659-3-jasowang@redhat.com


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