add exec(-status) to qm

add exec(-status) to qm

on the commandline the implementation for exec is a bit different
because there we want (by default) to wait for the result,
as opposed to the api, where it is enough to return the pid and
let the client handle the polling

this behaviour is optional and can be turned off, as well as the
timeout of 30 seconds

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