protect the scsi controller to be deleted when it is in use

protect the scsi controller to be deleted when it is in use

Steps to reproduce this bug:
1. virsh attach-disk domain --source imagefile --target sdb --sourcetype file --driver qemu --subdriver raw
2. virsh detach-device controller.xml # remove scsi controller 0
3. virsh detach-disk domain sdb
   error: Failed to detach disk
   error: operation failed: detaching scsi0-0-1 device failed: Device 'scsi0-0-1' not found

I think we should not detach a controller when it is used by some other device.

Signed-off-by: Wen Congyang <wency@cn.fujitsu.com>