qemuxml2argvtest: Add parsing of the input XML as separate test

qemuxml2argvtest: Add parsing of the input XML as separate test

Get clean separation between the parsing and argv conversion so that
it's obvious in the test output:

2409) QEMU XML def parse s390-async-teardown.s390x-6.0.0                ... libvirt: QEMU Driver error : unsupported configuration: asynchronous teardown is not available with this QEMU binary
OK
2410) QEMU XML def -> ARGV s390-async-teardown.s390x-6.0.0              ... SKIP

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