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.
04100cd281