Use fully qualified volume paths
Implement 'instances' method for all LVM resources

This method makes it possible to use constructs like:
  resources { 'physical_volume': purge => true, noop => true, }
  resources { 'logical_volume':  purge => true, noop => true, }
  resources { 'volume_group':    purge => true, noop => true, }
Implement empty 'extents' method

It is required by 'instances' method.
There is no simple way to get LV size in extents,
so leave it empty for now.
Set default LV unit size to MB