tests: add only directive for architectures

Some tests are x86-specific.  Now that all guests call architectures the
same way in the tuples, filter them away.
Signed-off-by: NPaolo Bonzini <pbonzini@redhat.com>

qemu/tests/cfg/physical_resources_check.cfg

 - physical_resources_check: install setup image_copy unattended_install.cdrom
+    only i386, x86_64
     type = physical_resources_check
     default_cpu_model = "qemu64"
     cpu_model_vendor = "AuthenticAMD"

qemu/tests/cfg/qemu_cpu.cfg

 - qemu_cpu:
+    only i386, x86_64
     virt_test_type = qemu
     type = cpuid
     start_vm = "no"