vDPA: report virtio-block discarding configuration to user space

This commit reports virtio-blk discarding configuration
to user space,includes:
1) the maximum discard sectors
2) maximum number of discard segments for the block driver to use
3) the alignment for splitting a discarding request

Signed-off-by: Zhu Lingshan <lingshan.zhu@intel.com>
Message-Id: <20240218185606.13509-8-lingshan.zhu@intel.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>