Can not create volume snapshot when using NfsDriver

Bug Description:
When I am using NfsDriver as my volume driver, I found that I can not create volume snapshot.

Bug Analysis:
I checked the NfsDriver and it doesn't contain the method for create_snapshot and create_volume_from_snapshot, by default, cinder use the parent class's methods.

Solution Analysis:
At least, we should add three function: create_snapshot, create_volume_from_snapshot and delete_snapshot.
create_snapshot can handle snapshot creation, It can call parent's _copy_volume method to copy from a base image to snapshot. But the progress is slow, I'm not sure how to optimize it.
create_volume_from_snapshot can handle create volume from snapshot, the same method as create_snapshot.
delete_snapshot can handle delete volume from snapshot, just remove the target file.

Error Logs:
2013-01-08 09:44:21 2133 TRACE cinder.volume.driver Command: sudo cinder-rootwrap /etc/cinder/rootwrap.conf lvdisplay cinder-volumes/_snapshot-0279ba1c-1caa-4f8c-aead-92d0f10aee8e
2013-01-08 09:44:21 2133 TRACE cinder.volume.driver Exit code: 5
2013-01-08 09:44:21 2133 TRACE cinder.volume.driver Stdout: ''
2013-01-08 09:44:21 2133 TRACE cinder.volume.driver Stderr: ' Volume group "cinder-volumes" not found\n Skipping volume group cinder-volumes\n'
2013-01-08 09:44:21 2133 TRACE cinder.volume.driver