Change create_instance to create PV-based as well as VG-based format instances.

Change create_instance to create PV-based as well as VG-based format instances.
Add supporting functions to work with the format instance and metadata area
structures stored within the format instance. Add support for simple indexing
of metadata areas using PV id and mda order (for on-disk PV only for now, we
can extend the indexing even for other mdas if needed - we only need to define
a proper key for the index).