this imitates the qemu-guest-agent interface
with an 'exec' api call which returns a pid
and an 'exec-status' api call which takes a pid
the command for the exec call is given as an 'alist'
which means that when using we have to give the 'command'
parameter multiple times e.g.
pvesh create <...>/exec --command ls --command '-lha' --command '/home/user'
so that we avoid having to deal with shell escaping etc.
Signed-off-by: Dominik Csapak <d.csapak@proxmox.com>