Fix dumpxml default value for s390x

On s390x cpu match "minimum" is not supported.
Set default "exact" if not subtest "with_cpu" to avoid tests being skipped.
Signed-off-by: NSebastian Mitterle <smitterl@redhat.com>

libvirt/tests/cfg/virsh_cmd/domain/virsh_dumpxml.cfg

 - virsh.dumpxml:
     type = virsh_dumpxml
     start_vm = "yes"
+    s390-virtio:
+        cpu_mode=exact
     variants:
         - normal_test:
             status_error = "no"