vmm: Exit VMM event loop after guest shutdown for AArch64

X86 and AArch64 work in different ways to shutdown a VM.
X86 exit VMM event loop through ACPI device;
AArch64 need to exit from CPU loop of a SystemEvent.

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

vmm/src/lib.rs

@@ -365,7 +365,8 @@ impl Vmm {
         #[cfg(not(feature = "acpi"))]
         {
             if self.vm.is_some() {
-                return self.vm_shutdown();
+                self.exit_evt.write(1).unwrap();
+                return Ok(());
             }
         }