tests: Reuse qemucapabilities data for qemucaps2xml

tests: Reuse qemucapabilities data for qemucaps2xml

While qemucaps2xml has a meager two test cases to its name, we
have plenty of data from qemucapabilities which is taken from
actual QEMU binaries, covers pretty much all supported QEMU
versions and architectures and is even in the right format already!

Rewrite qemucaps2xml so that it uses qemucapabilities data as
input. Right now we have a single test case, but we're going to
add a lot more next.

Signed-off-by: Andrea Bolognani <abologna@redhat.com>
Reviewed-by: Jiri Denemark <jdenemar@redhat.com>