vhost-blk: turn on pre-defined RO feature bit

vhost-blk: turn on pre-defined RO feature bit

Read only feature shouldn't be negotiable, because if the
backend device reported Read only feature supported, QEMU
host driver shouldn't change backend's RO attribute. While
here, also enable the vhost-user-blk test utility to test
RO feature.

Signed-off-by: Changpeng Liu <changpeng.liu@intel.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>