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.
Merge pull request #81 from aplanas/master

action #2856 - Makes the margin related with the needle
make typing more reliable

I moved the actual typing into the backend thread, so the thread can check
that after every keystroke the screen changes. This is much more reliable
than blindly sleeping
doc: hooks documentation
Merge pull request #82 from aaannz/pre-post-scripts

Pre&Post hooks for tests
Merge pull request #83 from os-autoinst/vnc

make typing more reliable
Add a new test case for large margins.
Increase the default value of the margin.