If a SCSI hostdev is included in an initial domain XML, without a
corresponding controller statement, one is created silently when the
guest is booted.

When hotplugging a SCSI hostdev, a presumption is that the controller
is already present in the domain either from the original XML, or via
an earlier hotplug.

  [root@xxxxxxxx ~]# cat disk.xml
  <hostdev mode='subsystem' type='scsi'>
    <source>
      <adapter name='scsi_host0'/>
      <address bus='0' target='3' unit='1088438288'/>
    </source>
  </hostdev>
  [root@xxxxxxxx ~]# virsh attach-device guest01 disk.xml
  error: Failed to attach device from disk.xml
  error: internal error: unable to execute QEMU command 'device_add': Bus 'scsi0.0' not found

Since the infrastructure is in place, we can also create a controller
silently for use by the hotplugged hostdev device.
Signed-off-by: NEric Farman <farman@linux.vnet.ibm.com>