qemu image format parameter

Currently it is not possible to specify the format-parmater.

If you have a raw image:

`image_path = "asdf.img"`

qemu does output the following warning:

```
WARNING: Image format was not specified for 'asdf.img' and probing guessed raw.
Automatically detecting the format is dangerous for raw images, write operations on block 0 will be restricted.
Specify the 'raw' format explicitly to remove the restrictions.
```

`image_path = "asdf.img,format=raw"`

does not work because you use the basename of image_path as the maschine name.

Thanks

Start by locating the QEMU driver code that parses image_path and uses its basename as the machine name. Trace how image paths are passed to QEMU, then verify that an image format such as raw can be specified without changing the machine name and that the resulting invocation removes the warning.

Written by the indexing model from the issue text.