Enable the floppy_test only on i440fx

This test only makes sense on x86 archs and it is currently disabled on
q35. Let's enable it only on i440fx then.
Signed-off-by: NLukáš Doktor <ldoktor@redhat.com>

qemu/tests/cfg/floppy_test.cfg

 - floppy_test: install setup image_copy unattended_install.cdrom
     type = floppy
-    no pseries
-    no q35
+    only i440fx
     start_vm = no
     floppies = "fl"
     guest_floppy_path = "/dev/fd0"