Description of problem:
Since we already support attach redirdev, it's better to fix detach redirdev. 

Version-Release number of selected component (if applicable):
libvirt-1.3.2-1.fc24_v1.3.2_rc2_1_g33fb8ff.x86_64

How reproducible:
100%

Steps to Reproduce:
1.start a guest with redirdev

# virsh dumpxml vm1|grep redirdev -A 3
    <redirdev bus='usb' type='spicevmc'>
      <alias name='redir0'/>
    </redirdev>

2. prepare a redirdev xml like following

# cat usb-redir.xml 
<redirdev bus='usb' type='spicevmc'>
    </redirdev>

3.detach redirdev device

# virsh detach-device vm1 usb-redir.xml 
error: Failed to detach device from usb-redir.xml
error: Operation not supported: live detach of device 'redirdev' is not supported

# virsh detach-device vm1 usb-redir.xml --config
error: Failed to detach device from usb-redir.xml
error: Operation not supported: persistent detach of device 'redirdev' is not supported

Actual results:
As step3, fail to detach it.

Expected results:
It could be detached. 

Additional info:

using QMP to check :
before delete, using virt-viewer to open guest we could find that table "Redirect USB device" is useful. 
then we detach it via QMP
virsh # qemu-monitor-command vm1 --pretty '{"execute":"device_del","arguments":{"id":"redir0"}}'
{
    "return": {

    },
    "id": "libvirt-25"
}

Then using virt-viewer to open guest to check again, we will find that table "Redirect USB device" is gray and it cannot be used.So actually it could be detached.