add pre- start/stop hookscript to containers

add pre- start/stop hookscript to containers

this adds the config (hookscript) and executes it on four points in
time for the container:

'pre-start'
'post-start'
'pre-stop'
'post-stop'

on pre-start we abort if the script fails and pre-stop will not be
called if the vm crashes or if the vm gets powered off from inside
the guest

Signed-off-by: Dominik Csapak <d.csapak@proxmox.com>