qemuxml2argvtest: Test (inactive) def -> xml conversion

qemuxml2argvtest: Test (inactive) def -> xml conversion

This is an intermediate step to merge qemuxml2xmltest into this common
helper. This eliminates double setup/parsing of the input data as well
as will ensure that all input XMLs are tested both for ARGV as well as
XML output. For now we skip tests that don't have an output XML to show
that the this does everything that qemuxml2xmltest does.

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