vDPA: report virtio-blk flush info to user space

This commit reports whether a virtio-blk device
support cache flush command to user space

Signed-off-by: Zhu Lingshan <lingshan.zhu@intel.com>
Message-Id: <20240218185606.13509-11-lingshan.zhu@intel.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>