[git]Virt-Manager cannot start any previously created virtual machine with Qemu commit bd306cfe: 'spicevmc' is not a valid char driver name
Description of problem:
With qemu built on commit bd306cfe, I'm unable to start a previously created VM. 

Because of both bug #463 and #474 I was blocked from building qemu from git for something like a week or so. My last built and working Qemu is based on commit 9bef7ea9d9.

Doing a git bissect won't be an easy task :(
Steps to reproduce:
1. Build qemu using commit bd306cfe
2. Launch Virt-Manager
3. Try to launch a previously created VM or try to boot a new one.
Additional information:
Every single time I tried to launch a VM, I get a dialog box with this error message:

```
Error starting domain: internal error: qemu unexpectedly closed the monitor: 2021-07-18T07:56:50.116480Z qemu-system-x86_64: -chardev spicevmc,id=charchannel1,name=vdagent: 'spicevmc' is not a valid char driver name

Traceback (most recent call last):
  File "/usr/share/virt-manager/virtManager/asyncjob.py", line 65, in cb_wrapper
    callback(asyncjob, *args, **kwargs)
  File "/usr/share/virt-manager/virtManager/asyncjob.py", line 101, in tmpcb
    callback(*args, **kwargs)
  File "/usr/share/virt-manager/virtManager/object/libvirtobject.py", line 57, in newfn
    ret = fn(self, *args, **kwargs)
  File "/usr/share/virt-manager/virtManager/object/domain.py", line 1329, in startup
    self._backend.create()
  File "/usr/lib/python3.9/site-packages/libvirt.py", line 1353, in create
    raise libvirtError('virDomainCreate() failed')
libvirt.libvirtError: internal error: qemu unexpectedly closed the monitor: 2021-07-18T07:56:50.116480Z qemu-system-x86_64: -chardev spicevmc,id=charchannel1,name=vdagent: 'spicevmc' is not a valid char driver name
```