The current syntax uses a pair of args

   -net nic,macaddr=52:54:00:56:6c:55,vlan=3,model=pcnet,name=pcnet.0
   -net user,vlan=3,name=user.0

The new syntax does not  need the vlan craziness anymore, and
so has a simplified pair of args

   -netdev user,id=user.0
   -device pcnet,netdev=user.0,id=pcnet.0,mac=52:54:00:56:6c:55,addr=<PCI SLOT>

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

When parsing the <disk> element specification, if no <address>
is provided for the disk, then automatically assign one based on
the <target dev='sdXX'/> device name. This provides for backwards
compatability with existing applications using libvirt, while also
allowing new apps to have complete fine grained control.

* src/conf/domain_conf.h, src/conf/domain_conf.c,
  src/libvirt_private.syms: Add virDomainDiskDefAssignAddress()
  for assigning a controller/bus/unit address based on disk target
* src/qemu/qemu_conf.c: Call virDomainDiskDefAssignAddress() after
  generating XML from ARGV
* tests/qemuxml2argvdata/*.xml: Add in drive address information
  to all XML files