Filter unittest_kvmctl to x86 only

Ideally this test should not exists in Avocado-vt as kvm-unit-tests are
dynamic and more and more are added each day. Anyway as it exists let's
limit it to x86 only as it defines x86 variants.

Those who want to run kvm-unit-tests please use pure Avocado instead.
Signed-off-by: NLukáš Doktor <ldoktor@redhat.com>

qemu/tests/cfg/unittest_kvmctl.cfg

 - unit_test_kvmctl:
     no JeOS
+    only i386, x86_64
     type = unittest_kvmctl
     vms = ''
     profilers = ''