vdpasim: allow to enable a vq repeatedly

Code must be resilient to enable a queue many times.

At the moment the queue is resetting so it's definitely not the expected
behavior.

v2: set vq->ready = 0 at disable.

Fixes: 2c53d0f6

 ("vdpasim: vDPA device simulator")
Cc: stable@vger.kernel.org
Signed-off-by: Eugenio Pérez <eperezma@redhat.com>
Message-Id: <20220519145919.772896-1-eperezma@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Reviewed-by: Stefano Garzarella <sgarzare@redhat.com>