vDPA: report virtio-block max segment size to user space

This commit allows reporting the max size of any
single segment of virtio-block devices to user space.

Signed-off-by: Zhu Lingshan <lingshan.zhu@intel.com>
Message-Id: <20240218185606.13509-3-lingshan.zhu@intel.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>