When you're done, issuing a reboot command inside the guest will actually shutdown Firecracker gracefully. This is due to the fact that Firecracker doesn't implement guest power management.
root@fadfdd4af58a:~# reboot [ OK ] Stopped target Timers. [ OK ] Stopped Daily Cleanup of Temporary Directories. [ OK ] Stopped Daily apt upgrade and clean activities. [ OK ] Stopped Daily apt download activities. [ OK ] Reached target Unmount All Filesystems. [ OK ] Stopped target Graphical Interface. [ OK ] Stopped target Multi-User System. Stopping OpenBSD Secure Shell server... [ OK ] Stopped target Login Prompts. Stopping Getty on tty2... Stopping Getty on tty3... [ OK ] Stopped getty on tty2-tty6 if dbus and logind are not available. Stopping Serial Getty on ttyS0... Stopping Getty on tty5... Stopping Getty on tty6... [ OK ] Stopped Message of the Day. Stopping Getty on tty4... [ OK ] Stopped Discard unused blocks once a week. [ OK ] Stopped OpenBSD Secure Shell server. [ OK ] Stopped Getty on tty2. [ OK ] Stopped Getty on tty6. [ OK ] Stopped Getty on tty3. [ OK ] Stopped Getty on tty4. [ OK ] Stopped Getty on tty5. [ OK ] Stopped Serial Getty on ttyS0. [ OK ] Removed slice system-serialx2dgetty.slice. [ OK ] Removed slice system-getty.slice. Stopping Permit User Sessions... [ OK ] Stopped Permit User Sessions. [ OK ] Stopped target Remote File Systems. [ OK ] Stopped target Basic System. [ OK ] Stopped target Sockets. [ OK ] Stopped target Paths. [ OK ] Stopped target System Initialization. [ OK ] Stopped Apply Kernel Variables. [ OK ] Stopped Load Kernel Modules. [ OK ] Stopped target Swap. Stopping Update UTMP about System Boot/Shutdown... [ OK ] Stopped target Local Encrypted Volumes. [ OK ] Stopped Dispatch Password Requests to Console Directory Watch. [ OK ] Stopped Forward Password Requests to Wall Directory Watch. Stopping Load/Save Random Seed... [ OK ] Stopped target Slices. [ OK ] Stopped Update UTMP about System Boot/Shutdown. [ OK ] Stopped Load/Save Random Seed. [ OK ] Stopped Create Volatile Files and Directories. [ OK ] Stopped target Local File Systems. [ OK ] Stopped target Local File Systems (Pre). [ OK ] Stopped Create Static Device Nodes in /dev. [ OK ] Stopped Remount Root and Kernel File Systems. [ OK ] Reached target Shutdown. [ OK ] Reached target Final Step. Starting Reboot... [10224.432836] reboot: Restarting system [10224.432836] reboot: Restarting system 2020-09-27T14:06:22.330258272 [anonymous-instance:ERROR:src/vmm/src/vstate.rs:1081] Unexpected exit reason on vcpu run: SystemEvent(2, 0)