Change TimeoutStopSec= to TimeoutSec=

A previous PR #231 ("Increased microvm default timeout stop to 2m30s") changed
the default TimeoutStopSec= to 150 to avoid the VMs almost finish booting, but
TimeoutStopSec=[1] is the amount of time systemd waits for the service to stop,
not start. Switch to TimeoutSec= to set both TimeoutStartSec= and
TimeoutStopSec=.

[1] https://www.freedesktop.org/software/systemd/man/latest/systemd.service.html#TimeoutStopSec=

Signed-off-by: Dummyc0m <xieyuanchu@gmail.com>

Author: Dummyc0m

nixos-modules/host/default.nix

@@ -239,7 +239,7 @@ in
           WorkingDirectory = "${stateDir}/%i";
           ExecStart = "${stateDir}/%i/current/bin/microvm-run";
           ExecStop = "${stateDir}/%i/booted/bin/microvm-shutdown";
-          TimeoutStopSec = 150;
+          TimeoutSec = 150;
           Restart = "always";
           RestartSec = "5s";
           User = user;