Pre&Post hooks for tests

This is sort of RFC and if you are even interested.

In our use case, we need to talk to external server to before QEMU VM is started (to set proper nfs exports, create pxe files, etc.) so in the first commit, support for prestart.pm is added. In the same way as main.pm is loaded.

The second commit add hook after VM is stopped, but before anything else is done to it (e.g. cleaned up). The purpose is to collect log files and other interesting files from VM HDD. Sometimes we had a test failure which prevented console switch and so we were unable to upload logs using the standard way, hence this hook.