qemu.tests.qemu_img: correct the configuration

Make qemu_img.amend compatible with new host version
Signed-off-by: NYanan Fu <yfu@redhat.com>

qemu/tests/cfg/qemu_img.cfg

@@ -109,7 +109,7 @@
             no vmdk raw qed
             subcommand = commit
         - amend:
-            only Host_RHEL.m7
+            no Host_RHEL.m6
             subcommand = amend
             start_vm = no
             images += " stg"