qemu driver: `image_path` should not be required

### Nomad version
`Nomad v1.0.3 (08741d9f2003ec26e44c72a2c0e27cdf0eadb6ee)`

### Operating system and Environment details
```
$ cat /etc/os-release
NAME="Ubuntu"
VERSION="20.04.2 LTS (Focal Fossa)"

$ apt-cache policy qemu-kvm
qemu-kvm:
Installed: 1:4.2-3ubuntu6.12
```

### Issue
I utilize iscsi for all volumes on my vms. The `image_path` input field appears to construct a `-drive file=$` argument to pass to qemu, but does so in a strange and non-obvious manner.

I don't see the point of this. I cannot set the `image_path` value to the value I would use if I were building the `-drive` arg directly (i.e. `-drive file=iscsi://{...}` -> `image_path = "iscsi://{...}"` does not work). Instead have to do something like `image_path = "-drive file=iscsi://{...}"`. When done like this, the way of providing the drive file format is either omitted entirely or stripped:

1. Prefixed: `image_path = "-drive format=raw,file=iscsi://{...}"` somehow strips the `format=raw` portion (or something, I don't know what, haven't looked at the code) and qemu gives me this error: `Image format was not specified for {iscsi lun description}`
2. Suffixed: `image_path = "-drive file=iscsi://{...},format=raw"` does...weird stuff, and I get the following error message: `qemu-system-x86_64: -name 7,format=raw": Invalid parameter 'format'` (7 is the LUN ID of this particular volume).

And when I try to omit it entirely:

```
2 errors occurred: * failed to parse config: * Missing required argument: The argument "image_path" is required, but no definition was found.
```

### Reproduction steps
try to deploy a vm with a non-local-file drive

### Job file (if appropriate)
...

### Nomad Client logs (if appropriate)
...

Ultimately some clarification on why `image_path` is required at all, or where it even really makes sense in a typical workflow, would be appreciated.

If a PR for this driver to stop requiring `image_path` would be accepted, I can work on one.

Start at the Nomad QEMU driver entry point and trace how image_path is validated and converted into QEMU drive arguments. Reproduce the reported iSCSI configuration and inspect the resulting arguments and validation error. Done means image_path is optional and a non-local iSCSI drive can be configured with its format preserved.

Written by the indexing model from the issue text.