vdpa_sim_blk: add support for VIRTIO_BLK_T_FLUSH

The simulator behaves like a ramdisk, so we don't have to do
anything when a VIRTIO_BLK_T_FLUSH request is received, but it
could be useful to test driver behavior.

Let's expose the VIRTIO_BLK_F_FLUSH feature to inform the driver
that we support the flush command.

Signed-off-by: Stefano Garzarella <sgarzare@redhat.com>
Message-Id: <20220811083632.77525-4-sgarzare@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>