libvirt-xml: add a method named 'add_device' into VMXML.

Signed-off-by: Dongsheng Yang <yangds.fnst@cn.fujitsu.com>
libvirt-xml: implement controller device.

Signed-off-by: Dongsheng Yang <yangds.fnst@cn.fujitsu.com>
libvirt: Add test for libvirt scsi.

Signed-off-by: Dongsheng Yang <yangds.fnst@cn.fujitsu.com>