Re: [libvirt] [PATCHv2 4/9] conf: set up for per-grammar overrides in schemas

This patch is my first experience playing with nested grammars, as documented in http://relaxng.org/tutorial-20011203.html#IDA3PZR. I plan on doing more overrides in order to make the RelaxNG grammar mirror the C code refactoring into a common virStorageSource, but where different clients of that source do not support the same subset of functionality. By starting with something fairly easy to validate, I can make sure my later patches will be possible. This patch adds a use of the no-op <ref name='sourceStartupPolicy'/> to the disksnapshot definition, so that the snapshot version of a type='file' <source> more closely resembles the version in domaincommon. A future patch will merge the two files into using a common define, but this patch is sufficient for testing that adding <source startupPolicy='optional'/> in any of the tests/domainsnapshotxml2xmlin/*.xml files still gets rejected unless it occurs within the <domain> subelement, because the definition of startupPolicy is empty outside of domain.rng.