The current code for using -drive simply sets the -drive 'index'
parameter. QEMU internally converts this to bus/unit depending
on the type of drive. This does not give us precise control over
the bus/unit assignment though. This change switches over to make
libvirt explicitly calculate the bus/unit number.

In addition bus/unit/index are actually irrelevant for VirtIO
disks, since each virtio disk is a separate PCI device. No disk
controller is involved.

Doing the conversion to bus/unit in libvirt allows us to correctly
attach SCSI controllers when required.

* src/qemu/qemu_conf.c: Specify bus/unit instead of index for
  disks
* tests/qemuxml2argvdata/qemuxml2argv-disk*.args: Switch over from
  using index=NNNN, to bus=NN, unit=NN for SCSI/IDE/Floppy disks