conf: move storage formats to common RNG file
    
    We had incomplete RelaxNG support for storage formats listed
    in virstoragefile.h: commit 027bf2e 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: Eric Blake <eblake@redhat.com>