vdpa_sim_blk: check if sector is 0 for commands other than read or write

VIRTIO spec states: "The sector number indicates the offset
(multiplied by 512) where the read or write is to occur. This field is
unused and set to 0 for commands other than read or write."

Signed-off-by: Stefano Garzarella <sgarzare@redhat.com>
Message-Id: <20220811083632.77525-2-sgarzare@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>