vdpa_sim_blk: allocate the buffer zeroed

Deleting and recreating a device can lead to having the same
content as the old device, so let's always allocate buffers
completely zeroed out.

Fixes: abebb162 ("vdpa_sim_blk: support shared backend")
Suggested-by: Qing Wang <qinwang@redhat.com>
Signed-off-by: Stefano Garzarella <sgarzare@redhat.com>
Message-Id: <20231031144339.121453-1-sgarzare@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Acked-by: Eugenio Pérez <eperezma@redhat.com>
Acked-by: Jason Wang <jasowang@redhat.com>