add hook to run script before testing begin

- in some cases (e.g. pxe boot testing), there may be a need to
execute some initialization steps before QEMU starts VM.
- This patch adds support for prestart.pm file inside test case
root directory. When this file is detected, it is executed before
VM is created.
add hook to run script after testing finish

- this patch adds support for postrun.pm file inside test case
root directory. When this file is detected, it is executed after
VM is stopped.
doc: hooks documentation
Merge pull request #82 from aaannz/pre-post-scripts

Pre&Post hooks for tests