[git] "qemu-system-x86_64: Parameter 'drive' is missing" when I tried to launch an existing VM in Virt-Manager.
Description of problem:
This bug is related in some way to bug #488.

I cannot start an existing virtual machine using qemu-git.
Additional information:
```
internal error: process exited while connecting to monitor: 2021-07-19T19:24:27.044654Z qemu-system-x86_64: Parameter 'drive' is missing

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: process exited while connecting to monitor: 2021-07-19T19:24:27.044654Z qemu-system-x86_64: Parameter 'drive' is missing

```

My last working build was made using commit 9bef7ea9. Using Peter Maydell commits as milestone, I noticed commit 9aef0954 was the first showing the bug.

I'll try to do bisect between these two commits and report asap. There is about 40 commits to verify.