spice: Restrict it to i386 and x86_64

This test uses hardcoded i386 and x86_64 machine types, let's restrict
it to those archs only.
Signed-off-by: NLukáš Doktor <ldoktor@redhat.com>

qemu/tests/cfg/spice.cfg

 - rv:
     no JeOS
+    only i386, x86_64
     vms += " vm2"
     guest_vm = virt-tests-vm1
     client_vm = vm2