virschematest: Improve detection of 'invalid' XMLs

virschematest: Improve detection of 'invalid' XMLs

The output files from 'qemuxml2argvtest' may have the real capability
suffix e.g. 'pci-rom-disabled-invalid.x86_64-latest.xml' which would not
be detected as being invalid and thus causing a test failure.

Change the logic to find '-invalid.' so that we can properly use
'virschematest' with test cases using real capabilities.

Signed-off-by: Peter Krempa <pkrempa@redhat.com>
Reviewed-by: Ján Tomko <jtomko@redhat.com>