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 <59c7438a9275f5dd0b96a91958b493a9d005ad46@cn.fujitsu.com>

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