Filter seabios test to x86 only

This test only makes sense in x86 world. Let's use only instead of out.
Signed-off-by: NLukáš Doktor <ldoktor@redhat.com>

qemu/tests/cfg/seabios.cfg

 - seabios: install setup image_copy unattended_install.cdrom
     no Host_RHEL.m5
-    no ppc64 ppc64le aarch64
+    only i386, x86_64
     type = seabios
     start_vm = no
     restart_vm = no