shared.cfg: Disable S3/S4 test on all ppc64 guests

According to https://github.com/autotest/virt-test/pull/558
All S3/S4 tests should be disabled on ppc64 guests.

Signed-off-by: Qingtang Zhou <qzhou@redhat.com>
Merge pull request #566 from zhouqt/no-suspend-on-ppc

shared.cfg: Disable S3/S4 test on all ppc64 guests