vmm: Implement VM rebooting on AArch64

The logic to handle AArch64 system event was: SHUTDOWN and RESET were
all treated as RESET.

Now we handle them differently:
- RESET event will trigger Vmm::vm_reboot(),
- SHUTDOWN event will trigger Vmm::vm_shutdown().

Signed-off-by: Michael Zhao <michael.zhao@arm.com>

hypervisor/src/cpu.rs

@@ -194,6 +194,7 @@ pub enum VmExit<'a> {
     MmioWrite(u64 /* address */, &'a [u8]),
     Ignore,
     Reset,
+    Shutdown,
     Hyperv,
 }
 

hypervisor/src/kvm/mod.rs

@@ -738,10 +738,10 @@ impl cpu::Vcpu for KvmVcpu {
                     use kvm_bindings::{KVM_SYSTEM_EVENT_RESET, KVM_SYSTEM_EVENT_SHUTDOWN};
                     // On Aarch64, when the VM is shutdown, run() returns
                     // VcpuExit::SystemEvent with reason KVM_SYSTEM_EVENT_SHUTDOWN
-                    if event_type == KVM_SYSTEM_EVENT_SHUTDOWN
-                        || event_type == KVM_SYSTEM_EVENT_RESET
-                    {
+                    if event_type == KVM_SYSTEM_EVENT_RESET {
                         Ok(cpu::VmExit::Reset)
+                    } else if event_type == KVM_SYSTEM_EVENT_SHUTDOWN {
+                        Ok(cpu::VmExit::Shutdown)
                     } else {
                         Err(cpu::HypervisorCpuError::RunVcpu(anyhow!(
                             "Unexpected system event with type 0x{:x}, flags 0x{:x}",