qemu: call post-parse callbacks when parsing command line too

Now we set the default disk driver name when parsing
the qemu command line too, hence all the test changes.

Assume format type is 'auto' when none is specified on
qemu command line.

tests/qemuxml2argvdata/qemuxml2argv-graphics-vnc-tls.xml

@@ -15,6 +15,7 @@
   <devices>
     <emulator>/usr/bin/qemu</emulator>
     <disk type='block' device='disk'>
+      <driver name='qemu'/>
       <source dev='/dev/HostVG/QEMUGuest1'/>
       <target dev='hda' bus='ide'/>
       <address type='drive' controller='0' bus='0' target='0' unit='0'/>