qemu-vm: Add option to create empty disk images.

This allows to add additional raw disk images to the VM, which therein are
available as /dev/vdb, /dev/vdc, /dev/vde and so on. Especially when testing
partitioning, this could be useful.

Signed-off-by: aszlig <aszlig@redmoonstudios.org>
tests: Add VM test for partitioning using nixpart.

At the moment, we still use kickstart syntax, but this is going to change
soon[TM] to be more NixOS-integrated. The tests use emptyDiskImages option
introduced in the previous commit, so we don't have to create a whole bunch of
duplicate expressions.

Testing itself is done exclusively on /dev/vdb and /dev/vdc. And there is a
check (ensureSanity) to make sure none of the tests are actually mutating
/dev/vda.

Signed-off-by: aszlig <aszlig@redmoonstudios.org>