libvirt-enable suppport for discard option for cinder-volume managed by nova

This blueprint aims to support specifying the discard option for "cinder volume" when it is attached to VM by nova.

[Background]
Supporting for discard option for disks managed by nova is already implemented.
(please see this :
  http://specs.openstack.org/openstack/nova-specs/specs/juno/implemented/libvirt-disk-discard-option.html)
And this feature only can support for nova instance not cinder volume.
It means that supporting for discard option for "cinder volume" is not implemented yet.
When cinder volume is created as thin provisioned, it should be attach with discard option for reclaim.

[Benefit for the user]
1.
Useful for cinder volume to recycle the unused block in thin provision case.

[Usage]
Support the user to specify the discard metadata for a volume when attach volume to VM.
Such as:
"
1. create cinder volume
2. update metadata of cinder volume
   ex) cinder metadata [volume uuid] set hw_disk_discard=unmap/ignore
3. attach volume to VM
   ex) nova volume-attach [vm uuid] [volume-uuid]
"

[Todo]
1. support discard option for cinder volume when attach volume to VM with cinder metadata.