Previously you would be able to override only the QEMU package to be
used in the test runner. Frankly that doesn't help a lot if you are
trying to get a graphical session. The graphical session requires the
option in the NixOS module system to bet set to the correct QEMU
package.

In this commit I moved most of the test node configuration and
transformations into the `mkDriver` function (previously called
`driver`). The motivation was to be able to create a `driver` instance
with a given QEMU package that will be used consistently througout the
test expression.

In a previous commit I broke this as there is no longer one testDriver
but only a function to generate one based on some QEMU inputs.

In 5500dc8 we introduced the --keep-vm-state flag and defaulted to that
flag not being set. This lead to the `runInMachine` tests not longer
working and that going unnoticed for quite some time now.

This reverts commit aab534b & uses the
driverInteractive attribute for the test driver instead.

This has the same effect but removes the extra module in the
nixos-build-vms code.

nixos/tests: follow-up to the closure reduction PR