domain conf: Track <console> target type

All <console> devices now export a <target> type attribute. QEMU defaults
to 'serial', UML defaults to 'uml, xen can be either 'serial' or 'xen'
depending on fullvirt. Understandably there is lots of test fallout.

This will be used to differentiate between a serial vs. virtio console for
QEMU.

Signed-off-by: Cole Robinson <crobinso@redhat.com>

tests/qemuxml2xmloutdata/qemuxml2xmlout-console-compat-auto.xml

@@ -24,7 +24,7 @@
       <target port='0'/>
     </serial>
     <console type='pty'>
-      <target port='0'/>
+      <target type='serial' port='0'/>
     </console>
     <memballoon model='virtio'/>
   </devices>