conf: also mark the implicit video as primary

Commit 119cd06 started setting the primary bool for the first
user-specified video even if user omitted the 'primary' attribute.

However this was done before the addition of the implicit device.
This broke startup of transient qemu domains with no <video>:
https://bugzilla.redhat.com/show_bug.cgi?id=1325757

Move this default to virDomainDefPostParseInternal,
after the addition of the implicit video device, to catch the implicit
video as well.

tests/vmx2xmldata/vmx2xml-scsi-writethrough.xml

@@ -19,7 +19,7 @@
     </disk>
     <controller type='scsi' index='0' model='buslogic'/>
     <video>
-      <model type='vmvga' vram='4096'/>
+      <model type='vmvga' vram='4096' primary='yes'/>
     </video>
   </devices>
 </domain>