conf: Correctly format controller's driver

conf: Correctly format controller's driver

https://bugzilla.redhat.com/show_bug.cgi?id=1179684

The way that we currently generate the <driver/> for <controller/> is
just madness:

    <controller type='scsi' index='0' model='virtio-scsi'>
      <driver queues='12'/>
      <driver cmd_per_lun='123'/>
      <address type='pci' domain='0x0000' bus='0x00' slot='0x04' function='0x0'/>
    </controller>

It's obvious that we should be aiming at the following:

    <controller type='scsi' index='0' model='virtio-scsi'>
      <driver queues='12' cmd_per_lun='123'/>
      <address type='pci' domain='0x0000' bus='0x00' slot='0x04' function='0x0'/>
    </controller>

Signed-off-by: Luyao Huang <lhuang@redhat.com>