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>

vmm/src/lib.rs

@@ -432,7 +432,8 @@ impl Vmm {
 
     fn vm_reboot(&mut self) -> result::Result<(), VmError> {
         // Without ACPI, a reset is equivalent to a shutdown
-        #[cfg(not(feature = "acpi"))]
+        // On AArch64, before ACPI is supported, we simply jump over this check and continue to reset.
+        #[cfg(all(target_arch = "x86_64", not(feature = "acpi")))]
         {
             if self.vm.is_some() {
                 self.exit_evt.write(1).unwrap();