adt-virt-qemu: Ensure that python3 or python is available

We need Python for the auxverb helper. Exit with a meaningful error message if it isn't.

Adjust the adt-virt-qemu.1 manpage accordingly.

Fix adt-setup-vm to install python3-minimal if no Python is already installed.

Thanks to Christian Kastner for the initial patch!

Closes: #777478