Disable cpu_model on s390x in valgrind_memalign

"-cpu" can only be set in KVM mode on s390x, but this test runs in TCG.
Let's disable it.
Signed-off-by: NLukáš Doktor <ldoktor@redhat.com>

qemu/tests/cfg/valgrind_memalign.cfg

@@ -10,6 +10,8 @@
     Ubuntu:
         valgrind_install_cmd = "apt-get install -y valgrind"
     s390x:
+        # -cpu can't be set in TCG
+        cpu_model = ""
         # s390 firmware quits when no-bootable devices (unlike x86)
         extra_params += " -no-shutdown"
         expected_status = "paused (shutdown)"