Move lvm vg down to per-fs node

Instead of using the host's 'vol' attribute to determine the volume
group to use, create an optional attribute under each 'fs', called
'vg' which specifies the volume group for that particular filesystem.
This allows a given host backup to span more than a single volume
group.
Backup 'xattrs'

Check each file during the backup, and save any extended attributes
associated with it.  Restore does not pay attention to these, yet,
however.