If target image is a block device, access it directly

[omnivagant]

This will allow a user to specify a block device, like an LVM volume or ZFS zvol, as target.

[jirutka]

This is a nice idea, but it’s incomplete – cleanup function (alpine-make-vm-image:143-146) must be modified.

[omnivagant]

Like so?

In some sense I feel like I'm a bit lazy here in keeping the $nbd_dev name even if it isn't.

[jirutka]

Yes, there are more things that should be renamed and documented to make sense after this change.

[omnivagant]

What if we just call it $disk_dev instead, would that be enough?

[jirutka]

Yes, that’s fine, but the help message needs to be updated as well.

[omnivagant]

Of course, like so?

[jirutka]

Yes, thanks!