vDPA: report virtio-block max segments in a request to user space

This commit allows vDPA reporting the maximum number of
segments in a request of virtio-block devices to
user space.

Signed-off-by: Zhu Lingshan <lingshan.zhu@intel.com>
Message-Id: <20240218185606.13509-5-lingshan.zhu@intel.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>