Corrupt qcow2 images with broken bitmap unfixable
Description of problem:
During a backup of a VM (via bitmaps), the disk of the VM/Snapshot went out of space.
The VM was stopped, leaving the image in a bad state.

But now when trying to repair it, it was stuck:
```
# qemu-img check -r all /dev/mapper/e1d2ff33--c3fd--4c1a--bcd1--2047e4efc362-efbd8056--720a--47b6--bede--4325d576ffb9
qemu-img: Could not open '/dev/mapper/e1d2ff33--c3fd--4c1a--bcd1--2047e4efc362-efbd8056--720a--47b6--bede--4325d576ffb9': Bitmap '' doesn't satisfy the constraints
```

But if you want to remove the bitmap:
```
# qemu-img bitmap --remove /dev/mapper/e1d2ff33--c3fd--4c1a--bcd1--2047e4efc362-efbd8056--720a--47b6--bede--4325d576ffb9 ''
qemu-img: Could not open '/dev/mapper/e1d2ff33--c3fd--4c1a--bcd1--2047e4efc362-efbd8056--720a--47b6--bede--4325d576ffb9': qcow2: Image is corrupt; cannot be opened read/write
```

It seems like qemu-img check needs some option to clear invalid bitmaps. So the image can be repaired including dropping the invalid bitmap.