seclabel: make code and RNG match

seclabel: make code and RNG match

Commit b170eb99 introduced a bug: domains that had an explicit
<seclabel type='none'/> when started would not be reparsed if
libvirtd restarted.  It turns out that our testsuite was not
exercising this because it never tried anything but inactive
parsing.  Additionally, the live XML for such a domain failed
to re-validate.  Applying just the tests/ portion of this patch
will expose the bugs that are fixed by the other two files.

* docs/schemas/domaincommon.rng (seclabel): Allow relabel under
type='none'.
* src/conf/domain_conf.c (virSecurityLabelDefParseXML): Per RNG,
presence of <seclabel> with no type implies dynamic.  Don't
require sub-elements for type='none'.
* tests/qemuxml2xmltest.c (mymain): Add test.
* tests/qemuxml2argvtest.c (mymain): Likewise.
* tests/qemuxml2argvdata/qemuxml2argv-seclabel-none.xml: Add file.
* tests/qemuxml2argvdata/qemuxml2argv-seclabel-none.args: Add file.
Reported by Ansis Atteka.