qemu.test: enforce_quit.unknow_flag: disable this case on RHEL.6 host

guest didn't enforce quit while flag lacked in RHEL.6 host, and this problem
just fix on RHEL.7 host, so disable this case on RHEL.6 host.
Signed-off-by: NXueqiang Wei <xuwei@redhat.com>

qemu/tests/cfg/enforce_quit.cfg

@@ -24,6 +24,7 @@
             cpu_model = "Opteron_G1"
             cpu_model_flags = ",+svm,enforce"
         - unknow_flag:
+            no Host_RHEL.m6
             cpu_model_flags = ",-svm,+unknown,enforce"
         - unsupport_flag:
             cpu_model_flags = ",-svm,+sse4a,+xsave,+aes,enforce"