tests/qtest/migration-test: Enable on ppc64 TCG

tests/qtest/migration-test: Enable on ppc64 TCG

ppc64 with TCG seems to no longer be failing this test, perhaps since
commit 03bfc2188f061 ("physmem: Fix migration dirty bitmap coherency
with TCG memory access") which is not ppc specific but was seen to hit
ppc64 quite easily.

Let's enable it again.

The s390x problem has been identified so mention it while we are
adjusting the comment.

Reviewed-by: Thomas Huth <thuth@redhat.com>
Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
Reviewed-by: Prasad Pandit <pjp@fedoraproject.org>
Signed-off-by: Fabiano Rosas <farosas@suse.de>