conf: create common storage RNG grammar file

conf: create common storage RNG grammar file

Having two tiny files with a couple definitions didn't make
as much sense as one common file, especially since I plan to
add more definitions and use it in more places.

* docs/schemas/storageencryption.rng: Merge this...
* docs/schemas/storagefilefeatures.rng: ...and this, into...
* docs/schemas/storagecommon.rng: ...this new file.
* docs/schemas/Makefile.am (schema_DATA): Reflect renames.
* docs/schemas/storagevol.rng: Likewise.
* docs/schemas/domaincommon.rng: Likewise.
* libvirt.spec.in: Likewise.
* mingw-libvirt.spec.in: Likewise.

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