testQemuDiskXMLToProps: Store all per-image data in one structure

testQemuDiskXMLToProps: Store all per-image data in one structure

We had two non-syncrhonized arrays holding the individual data. This was
a lazy way to do it when I was adding new tests recently. Since it's
hard to extend with new data to test refactor the storage of test data
to use a new struct where all per-image data are kept and can be
extended easily.

Signed-off-by: Peter Krempa <pkrempa@redhat.com>
Reviewed-by: Michal Privoznik <mprivozn@redhat.com>