Disable smbios test on s390x

There are no plans for supporting the "-smbios" argument, nor there is
support for `dmidecode` on s390x.
Signed-off-by: NLukáš Doktor <ldoktor@redhat.com>

qemu/tests/cfg/smbios.cfg

 - smbios_table: install setup image_copy unattended_install.cdrom
     only Linux
+    # dmidecode not available (and no plans for it) on s390x
+    no s390x
     type = smbios_table
     requires_root = yes
     start_vm = no