vDPA: code clean for vhost_vdpa uapi

This commit cleans up the uapi for vhost_vdpa by
better naming some of the enums which report blk
information to user space, and they are not
in any official releases yet.

Fixes: 1ac61ddf ("vDPA: report virtio-blk flush info to user space")
Fixes: ae1374b7 ("vDPA: report virtio-block read-only info to user space")
Fixes: 330b8aea ("vDPA: report virtio-block max segment size to user space")
Signed-off-by: Zhu Lingshan <lingshan.zhu@intel.com>
Message-Id: <20240415111047.1047774-1-lingshan.zhu@intel.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>