qemu/tests:Disable s3 and s4 tests for ppc64

ppc64 platform does not support s3 and s4 cases.
So disable them when in ppc64 platform.

Signed-off-by: Mike Qiu <qiudayu@linux.vnet.ibm.com>
Merge pull request #558 from QiuMike/qemu_tests_Disable_s3_and_s4_tests_for_ppc64

qemu/tests:Disable s3 and s4 tests for ppc64