virt-build-kata: don't hardcode the disk image name

[fidencio]

This script could be used for spawning several VMs in parallel, which would result in errors in case the disk is not set by the user.

Here, I'd recommend a few things:

1. Instead of kata.qcow2, use kata-$d.qcow2 by default, as this would already allow users to have different distros running in parallel;
2. Check whether $disk already exists in the system, just to avoid overwriting something without advertising the user;

Note, I thought about trying a pseudo smart approach of naming the image kata-$d, checking for the existence, and creating a kata-$d-nn instead. However, I sincerely think it's over-engineering the problem at this point and I'd be way happier with only 1 and 2 being implemented.

[alicefr]

I put the disk image as a mandatory parameter. So, it's the responsibility of the user to set the name and path