schema: rewrite seclabel rng to match code

schema: rewrite seclabel rng to match code

The RNG for <seclabel> was too strict - if it was present, then it
had to have sub-elements, even if those didn't make sense for the
given attributes.  Also, we didn't have any tests of <seclabel>
parsing or XML output.

In this patch, I added more parsing tests than output tests (since
the output populates and/or reorders fields not present in certain
inputs).  Making the RNG reliable is a precursor to using <seclabel>
variants in more places in the XML in later patches.

See also:
http://berrange.com/posts/2011/09/29/two-small-improvements-to-svirt-guest-configuration-flexibility-with-kvmlibvirt/

* docs/schemas/domaincommon.rng (seclabel): Tighten rules.
* tests/qemuxml2argvtest.c (mymain): New tests.
* tests/qemuxml2xmltest.c (mymain): Likewise.
* tests/qemuxml2argvdata/qemuxml2argv-seclabel-*.*: New files.