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,
 }