storage: expose volume meta-type in XML

storage: expose volume meta-type in XML

I got annoyed at having to use both 'virsh vol-list $pool --details'
AND 'virsh vol-dumpxml $vol $pool' to learn if I had populated
the volume correctly.  Since two-thirds of the data present in
virStorageVolGetInfo() already appears in virStorageVolGetXMLDesc(),
this just adds the remaining piece of information, as:

<volume type='...'>
  ...
</volume>

* docs/formatstorage.html.in: Document new <volume type=...>.
* docs/schemas/storagevol.rng (vol): Add it to RelaxNG.
* src/conf/storage_conf.h (virStorageVolTypeToString): Declare.
* src/conf/storage_conf.c (virStorageVolTargetDefFormat): Output
the metatype.
(virStorageVolDefParseXML): Parse it, for unit tests.
* tests/storagevolxml2xmlout/vol-*.xml: Update tests to match.

Signed-off-by: Eric Blake <eblake@redhat.com>