Description of problem:
For a shutoff guest which has a scsi disk plug on a special scsi controller.
I could detached the useful scsi controller successfully, then I want to re-attach it again, it fails to attach.
And also I try to start guest after I detached the controller, at the first time it fails to start,
but it will add a new scis controller automatically;then trying to start guest again, it could start successfully.

Version-Release number of selected component (if applicable):
libvirt-1.3.2-1.fc24_v1.3.1_51_g356e28b.x86_64

How reproducible:
100%

Steps to Reproduce:
1. For a shut off guest as following, it has scsi disk sde use scis1 controller.
# virsh dumpxml vm1 | grep scsi -A 5
   <disk>
...
      <target dev='sde' bus='scsi'/>
      <address type='drive' controller='1' bus='0' target='0' unit='0'/>
    </disk>
    <controller type='scsi' index='0' model='virtio-scsi'>
      <address type='pci' domain='0x0000' bus='0x00' slot='0x03' function='0x0'/>
    </controller>
    <controller type='scsi' index='1' model='virtio-scsi'>
      <driver queues='3' cmd_per_lun='10' max_sectors='512' ioeventfd='on'/>
      <address type='pci' domain='0x0000' bus='0x00' slot='0x0a' function='0x0'/>
    </controller>

2. Detach scsi controller. check domain xml scsi1 controller was detached.

# cat controller/scsi-ctl-driver.xml
<controller type='scsi' index='1' model='virtio-scsi'>
<driver queues='3' cmd_per_lun='10' max_sectors='512' ioeventfd='on'/>
<address type='pci' domain='0x0000' bus='0x00' slot='0x0a' function='0x0'/>
</controller>

# virsh detach-device vm1 controller/scsi-ctl-driver.xml --config
Device detached successfully

# virsh dumpxml vm1 | grep scsi -A 5
      <target dev='sde' bus='scsi'/>
      <address type='drive' controller='1' bus='0' target='0' unit='0'/>
    </disk>
    <controller type='scsi' index='0' model='virtio-scsi'>
      <address type='pci' domain='0x0000' bus='0x00' slot='0x03' function='0x0'/>
    </controller>

3. try to re-attach the scsi controller back, it fails to re-attach.
# virsh attach-device vm1 controller/scsi-ctl-driver.xml --config
error: Failed to attach device from controller/scsi-ctl-driver.xml
error: Requested operation is not valid: Target already exists


4.check domain xml again, only one scsi controller.

# virsh dumpxml vm1 |grep scsi -A 5
      <target dev='sde' bus='scsi'/>
      <address type='drive' controller='1' bus='0' target='0' unit='0'/>
    </disk>

    <controller type='scsi' index='0' model='virtio-scsi'>
      <address type='pci' domain='0x0000' bus='0x00' slot='0x03' function='0x0'/>
    </controller>

5. try to start guest at first time
# virsh start vm1
error: Failed to start domain vm1
error: internal error: Could not find scsi controller with index 1 required for device

check domain xml.
# virsh dumpxml vm1 |grep scsi -A 5
      <target dev='sde' bus='scsi'/>
      <address type='drive' controller='1' bus='0' target='0' unit='0'/>
    </disk>
 
    <controller type='scsi' index='0' model='virtio-scsi'>
      <address type='pci' domain='0x0000' bus='0x00' slot='0x03' function='0x0'/>
    </controller>
--
    <controller type='scsi' index='1'/>   <== add a new scsi controller.

6. try to start guest again
# virsh start vm1
Domain vm1 started


Actual results:
As step 3 and step 5, after detach useful scsi controller, it fails to re-attach it again and fails to start guest at the first time.

Expected results:
Perhaps we should forbid to detach scsi controller when it's used by a disk even guest is shutoff.

Additional info:
It could be reproduced on rhel7.2.