tests: Unify data structure for vircaps2xmltest

If all we achieve is reducing the depth by one for a single
test case, the additional complexity (not to mention breaking
the principle of least surprise) is not worth it: let's use
simpler, more predictable code instead.

This basically reverts fec6e4c4 (with a few adjustments).
Signed-off-by: NAndrea Bolognani <abologna@redhat.com>
Reviewed-by: NMartin Kletzander <mkletzan@redhat.com>