tests: Simplify some ppc64 tests

tests: Simplify some ppc64 tests

We can leave out things like USB controller, memballoon device,
kernel and initrd since they're not the focus of the tests.

Propagating some information from the output files back to the
input files makes it easier to compare them, as it reduces the
resulting diff, and in the case of the qemuxml2xml test for
memory-hotplug-ppc64-nonuma it allows us to convert the output
file into a symlink, since in the specific case the XML doesn't
change at all.

Signed-off-by: Andrea Bolognani <abologna@redhat.com>
Reviewed-by: Ján Tomko <jtomko@redhat.com>
Reviewed-by: Daniel Henrique Barboza <danielhb413@gmail.com>