conf: split <disk> schema into more pieces

conf: split <disk> schema into more pieces

To make <disk> schema more maintainable and to allow for moving the
pieces to a common file in the future. It relies on the ability to
override definitions as part of an include, set up in the previous
patch.

The diff is a bit hard to read, because it mixes reindentation
with refactoring; 'git diff -b --patience' may help.

* docs/schemas/domaincommon.rng (disk): Refactor into pieces.
(diskSource, diskSourceFile, diskSourceBlock, diskSourceDir)
(diskSourceVolume: New defines.
(diskSourceNetwork): Revise scope.
* docs/schemas/domainsnapshot.rng (disksnapshot): Adjust.
* tests/domainsnapshotxml2xmlin/disk-seclabel-invalid.xml,
tests/domainsnapshotxml2xmlin/disk-network-seclabel-invalid.xml: New
tests to check seclabel is forbidden in domain snapshot by schema.

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