vDPA: report virtio-block capacity to user space

This commit allows userspace to query capacity of
a virtio-block device.

Signed-off-by: Zhu Lingshan <lingshan.zhu@intel.com>
Message-Id: <20240218185606.13509-2-lingshan.zhu@intel.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>