add pre- start/stop hookscripts to VMs

add pre- start/stop hookscripts to VMs

this adds a new config option for it, and executes it on four
points in time:

'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>