testCompareXMLToArgvValidateSchema: Improve and fix helper for testing everything
testCompareXMLToArgvValidateSchema: Improve and fix helper for testing everything

The schema validator has a comment which allows checking all xml2argv
input files for schema validity by forcing the latest schema onto files
which don't have any schema. Fix it so that it works properly with the
caching introduced in previous commit.

Signed-off-by: Peter Krempa <2cf5c04c61aa466e4a47bfedc747d17279c72ffc@redhat.com>
Reviewed-by: Jonathon Jongsma <c7254805a17bd5c41e24c3a278fcbf3afcdfb0fa@redhat.com>

libvirt/libvirt,olafhering/libvirt,nertpinx/libvirt,crobinso/libvirt,crobinso/libvirt,zippy2/libvirt,olafhering/libvirt,jfehlig/libvirt,nertpinx/libvirt,nertpinx/libvirt,olafhering/libvirt,crobinso/libvirt,jfehlig/libvirt,nertpinx/libvirt,jfehlig/libvirt,jfehlig/libvirt,nertpinx/libvirt,zippy2/libvirt,libvirt/libvirt,zippy2/libvirt,libvirt/libvirt,crobinso/libvirt,libvirt/libvirt,zippy2/libvirt,olafhering/libvirt