tests: Fix use of virtio-serial for aarch64/virt

virtio-serial is an alias for virtio-serial-pci, which
should not have been used for a PCIe-less aarch64/virt
guest but it ended up being used anyway because the
virtio-mmio capability was missing and the algorithm
is buggy.

Fix the test case so that we can fix the algorithm next.

Signed-off-by: Andrea Bolognani <abologna@redhat.com>
Reviewed-by: Ján Tomko <jtomko@redhat.com>

tests/qemuxml2xmloutdata/mach-virt-console-virtio.xml

@@ -18,7 +18,9 @@
   <devices>
     <emulator>/usr/bin/qemu-system-aarch64</emulator>
     <controller type='usb' index='0' model='none'/>
-    <controller type='virtio-serial' index='0'/>
+    <controller type='virtio-serial' index='0'>
+      <address type='virtio-mmio'/>
+    </controller>
     <console type='pty'>
       <target type='virtio' port='0'/>
     </console>