Auto-add disk controllers based on defined disks

Existing applications using libvirt are not aware of the disk
controller concept. Thus, after parsing the <disk> definitions
in the XML, it is neccessary to create <controller> elements
to satisfy all requested disks, as per their defined drive
addresses

* src/conf/domain_conf.c, src/conf/domain_conf.h,
  src/libvirt_private.syms: Add virDomainDefAddDiskControllers()
  method for populating disk controllers, and call it after
  parsing disk definitions.
* src/qemu/qemu_conf.c: Call virDomainDefAddDiskControllers()
  when doing ARGV -> XML conversion
* tests/qemuxml2argvdata/qemuxml2argv*.xml: Add disk controller
  data to all data files which don't have it already

tests/qemuxml2argvdata/qemuxml2argv-disk-usb.xml

@@ -23,5 +23,6 @@
       <source file='/tmp/usbdisk.img'/>
       <target dev='sda' bus='usb'/>
     </disk>
+    <controller type='ide' index='0'/>
   </devices>
 </domain>