libvirt_xml: Add a del_device method into VMXML to remove a device.

Signed-off-by: Dongsheng Yang <yangds.fnst@cn.fujitsu.com>
libvirt_scsi: Add a case to test auto-attaching of scsi controller.

As new libvirt will auto-attach scsi controller for any scsi device,
this patch add a case to make sure the auto-attaching works well.

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