Only allow the qemu_guest_agent.isa_serial on x86 machines

The isa-serial is not available outside x86 world, let's change the
filter-out to filter-only.
Signed-off-by: NLukáš Doktor <ldoktor@redhat.com>

qemu/tests/cfg/qemu_guest_agent.cfg

@@ -134,8 +134,8 @@
         - isa_serial:
             # Windows guests don't support isa serial, disable it directly.
             no Windows
-            # ppc64 and ppc64le platform doesn't support isa serial
-            no ppc64 ppc64le
+            # isa-serial is only supported on x86
+            only i386, x86_64
             gagent_serial_type = isa
             serials += " org.qemu.guest_agent.0"
             gagent_start_cmd = "pgrep qemu-ga || qemu-ga -d -m isa-serial -p /dev/ttyS1"