tests/avocado: Fix ppc_hv_tests.py xorriso dependency guard

tests/avocado: Fix ppc_hv_tests.py xorriso dependency guard

For some reason the skipIf missing_deps() check fails to skip the test
if it comes after the skipUnless lines, causing an error running on
systems without xorriso.

Avocado implements skipUnless is just an inverted skipIf, so it's not
clear what the bug is or why this fixes it. For now it's enough to
get things working.

Resolves: https://gitlab.com/qemu-project/qemu/-/issues/2246
Fixes: c9cb496710758 ("tests/avocado: ppc add hypervisor tests")
Signed-off-by: Nicholas Piggin <npiggin@gmail.com>