We had incomplete RelaxNG support for storage formats listed
in virstoragefile.h: commit 027bf2ea added 'vdi' but forgot
to update the <volume> and <domain> xml lists; the <volume>
list was also missing 'fat' and 'vhd'.  Maintaining two lists
is a recipe for them getting out of sync, so make the list
common so that both contexts benefit the next time we add a
format in a single location.

* docs/schemas/domaincommon.rng (storageFormat): Move...
* docs/schemas/storagecommon.rng: ...here, and add vdi.
* docs/schemas/storagevol.rng (formatfile): Use common list.
Signed-off-by: NEric Blake <eblake@redhat.com>