virschematest: Add possibility to have exceptions from the '-invalid' suffix

virschematest: Add possibility to have exceptions from the '-invalid' suffix

The exception is needed in qemuxml2xmltest which is in one instance
testing update from an invalid config to a valid one. Currently the
compliance with the test is achieved via a hack.

As further patches will be simpler without the hack present we need a
way to invert the expected output in specific cases.

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