Allow manual choice of kernel

[obbardc]

Currently fakemachine searches for the kernel path in the system directories. To make fakemachine more useful to development cases, allow the backends to be able to choose which kernel to run (similar to #119) in a generic way.

To make this generic, we could modify the qemu backend to accept options (see #191) like kernel-path and modules-path which would point to the specific path. If those options are not passed, we would continue to search the default paths.