core: Propagate exit status via notify socket when running in VM

core: Propagate exit status via notify socket when running in VM

When running in a container, we can propagate the exit status of
pid1 as usual via the process exit status. This is not possible
when running in a VM. Instead, let's send EXIT_STATUS=%i via the
notify socket if one is configured. The user running the VM can then
pick up the exit status from the notify socket after the VM has shut
down.