vmm: add guest exit event path

Introduce a dedicated guest_exit_evt and a matching epoll dispatch
path for guest-triggered shutdowns.

This series is needed because managment software such as libvirt may
still need the Cloud Hypervisor process to stay alive after the guest
has shut down.
Today a guest-triggered shutdown can make the VMM disappear immediately,
which means the managment software can lose track of the VM run-state.

This must only apply to guest-triggered shutdowns. Fatal error paths
and other internal exit paths must keep using the existing VMM exit
handling.

For now GuestExit still calls vmm_shutdown(), so this commit only adds
the separate plumbing and keeps the current behavior unchanged.

On-behalf-of: SAP leander.kohler@sap.com
Signed-off-by: Leander Kohler <leander.kohler@cyberus-technology.de>

vmm/src/vm.rs

@@ -536,6 +536,7 @@ impl Vm {
         vm: Arc<dyn hypervisor::Vm>,
         exit_evt: EventFd,
         reset_evt: EventFd,
+        guest_exit_evt: EventFd,
         #[cfg(feature = "guest_debug")] vm_debug_evt: EventFd,
         seccomp_action: &SeccompAction,
         hypervisor: Arc<dyn hypervisor::Hypervisor>,
@@ -604,6 +605,7 @@ impl Vm {
             cpu_manager.clone(),
             exit_evt.try_clone().map_err(Error::EventFdClone)?,
             reset_evt,
+            guest_exit_evt,
             seccomp_action.clone(),
             numa_nodes.clone(),
             &activate_evt,
@@ -790,6 +792,7 @@ impl Vm {
         cpu_manager: Arc<Mutex<cpu::CpuManager>>,
         exit_evt: EventFd,
         reset_evt: EventFd,
+        guest_exit_evt: EventFd,
         seccomp_action: SeccompAction,
         numa_nodes: NumaNodes,
         activate_evt: &EventFd,
@@ -812,6 +815,7 @@ impl Vm {
             cpu_manager,
             exit_evt,
             reset_evt,
+            guest_exit_evt,
             seccomp_action,
             numa_nodes,
             activate_evt,
@@ -1247,6 +1251,7 @@ impl Vm {
         vm_config: Arc<Mutex<VmConfig>>,
         exit_evt: EventFd,
         reset_evt: EventFd,
+        guest_exit_evt: EventFd,
         #[cfg(feature = "guest_debug")] vm_debug_evt: EventFd,
         seccomp_action: &SeccompAction,
         hypervisor: Arc<dyn hypervisor::Hypervisor>,
@@ -1319,6 +1324,7 @@ impl Vm {
             vm,
             exit_evt,
             reset_evt,
+            guest_exit_evt,
             #[cfg(feature = "guest_debug")]
             vm_debug_evt,
             seccomp_action,