Description of problem:
currently it fails to hotplug usb redirection via libvirt, and for --live and --config options it gives different error info.
actually libvirt should support it. This is not only be reproduced on upstream, rhel7.2.

Version-Release number of selected component (if applicable):

How reproducible:
100%

Steps to Reproduce:

1. prepare a usb redirection device like following 
# cat usb-redir.xml 
<redirdev bus='usb' type='spicevmc'/>

2. attach this usb redir device to a running guest 
# virsh attach-device vm1 usb-redir.xml 
error: Failed to attach device from usb-redir.xml
error: internal error: unable to execute QEMU command 'device_add': Property 'usb-redir.chardev' can't find value 'charredir0'

3. attach this usb redir device to a running guest with --config 
# virsh attach-device vm1 usb-redir.xml --config
error: Failed to attach device from usb-redir.xml
error: Operation not supported: persistent attach of device 'redirdev' is not supported

Actual results:
As above step 2 and stpe 3

Expected results:
We should hotplug usb redirection device successfully.
And if it doesn't support, we should give same error message for --live and --config.

Additional info:

For a usb redirection device it corresponds to two qemu cli :
-chardev spicevmc,id=charredir0,name=usbredir 
-device usb-redir,chardev=charredir0,id=redir0 
So before we do "device_add usb-redir", we should do "chardev-add spicevmc" first.

and, using QMP we could attach a usb redirdev to guest.
virsh # qemu-monitor-command  vm1 --pretty '{"execute":"chardev-add","arguments":{"id":"charredir0","backend":{"type":"spicevmc","data":{"type":"usbredir"}}}}'
{
    "return": {

    },
    "id": "libvirt-101"
}

virsh # qemu-monitor-command vm1 --pretty '{"execute":"device_add","arguments":{"driver":"usb-redir","chardev":"charredir0","id":"redir0"}}'
{
    "return": {

    },
    "id": "libvirt-102"
}

virsh # qemu-monitor-command --hmp vm1 info usb
  Device 0.0, Port 1, Speed 1.5 Mb/s, Product USB Redirection Device

Using virt-viewer login guest, can use usb redirection to redirect usb device into guest.


libvirt log :
2016-01-13 06:15:36.505+0000: 26772: debug : virDomainAttachDevice:8305 : dom=0x7f8c308e9cf0, (VM: name=vm1, uuid=29f1a1e1-3f8b-48bb-a8aa-c2a5902f86a1), xml=<redirdev bus='usb' type='spicevmc'/>
......
2016-01-13 06:15:36.506+0000: 26772: debug : qemuMonitorAddDeviceWithFd:2858 : device=usb-redir,chardev=charredir0,id=redir0 fd=-1 fdname=<null>
2016-01-13 06:15:36.506+0000: 26772: debug : qemuMonitorAddDeviceWithFd:2861 : mon:0x7f8c2400c1e0 vm:0x7f8bf02190a0 json:1 fd:26
2016-01-13 06:15:36.506+0000: 26772: debug : qemuMonitorJSONCommandWithFd:291 : Send command '{"execute":"device_add","arguments":{"driver":"usb-redir","chardev":"charredir0","id":"redir0"},"id":"libvirt-100"}' for write with FD -1
2016-01-13 06:15:36.506+0000: 26772: info : qemuMonitorSend:998 : QEMU_MONITOR_SEND_MSG: mon=0x7f8c2400c1e0 msg={"execute":"device_add","arguments":{"driver":"usb-redir","chardev":"charredir0","id":"redir0"},"id":"libvirt-100"}^M
 fd=-1
......