Guest reboot and power-off both exit 0, so callers can't tell them apart

A guest reboot and a guest power-off end the VM the same way: the process exits with 0.

• HVF: PSCI SYSTEM_OFF and SYSTEM_RESET both return VcpuExit::Shutdown (hvf/src/lib.rs#L639-L644), which exits with FC_EXIT_CODE_OK (macos/vstate.rs#L485-L488).
• KVM: KVM_SYSTEM_EVENT_RESET and KVM_SYSTEM_EVENT_SHUTDOWN both become VcpuEmulation::Stopped (linux/vstate.rs#L1566-L1573).
• x86: the i8042 reset is the exit event, and it exits with FC_EXIT_CODE_OK (vmm/mod.rs#L457-L480).

A program running a long-lived guest (a desktop VM through krunkit, in my case) can't tell whether to start the VM again when the process exits, so after a guest reboot its user has to start it by hand.

Giving reset its own exit code by default would break libkrun's init. It ends every workload with reboot(RB_AUTOBOOT) (exec.rs#L107-L110), and reports the workload's exit code with an ioctl only when / is virtio-fs (exec.rs#L65-L76). With a block-device root, the vCPU's exit code becomes the process's, so a new default would turn those 0s into the new code.

Proposal: opt-in. krun_set_reset_exit_code(ctx_id, code) sets the process exit code for a guest reset; the default stays as it is. The exit code the guest sets through the ioctl still takes precedence. HVF, KVM and the i8042 path would all use it. krunkit would get a flag to pass it through.

Tested: macOS, v1.19.5, with a patch mapping SYSTEM_RESET to 32 (crosvm's VmReset): systemctl reboot in a Fedora guest exits 32, and systemctl poweroff exits 0.

Would this shape work? Or would you rather have something else, such as a callback, or a different default where no init is in use?