vmm: Change booting process to cover AArch64 requirements

Between X86 and AArch64, there is some difference in booting a VM:
- X86_64 can setup IOAPIC before creating any VCPU.
- AArch64 have to create VCPU's before creating GIC.

The old process is:
1. load_kernel()
    load kernel binary
    configure system
2. activate_vcpus()
    create & start VCPU's

So we need to separate "activate_vcpus" into "create_vcpus" and
"activate_vcpus" (to start vcpus only). Setup GIC and create FDT
between the 2 steps.

The new procedure is:
1. load_kernel()
    load kernel binary
    (X86_64) configure system
2. create VCPU's
3. (AArch64) setup GIC
4. (AArch64) configure system
5. start VCPU's

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

vmm/src/vm.rs

@@ -527,7 +527,6 @@ impl Vm {
         };
 
         let boot_vcpus = self.cpu_manager.lock().unwrap().boot_vcpus();
-        let _max_vcpus = self.cpu_manager.lock().unwrap().max_vcpus();
 
         #[allow(unused_mut, unused_assignments)]
         let mut rsdp_addr: Option<GuestAddress> = None;
@@ -601,6 +600,11 @@ impl Vm {
         }
     }
 
+    #[cfg(target_arch = "aarch64")]
+    fn configure_system(&mut self) -> Result<()> {
+        unimplemented!();
+    }
+
     pub fn shutdown(&mut self) -> Result<()> {
         let mut state = self.state.try_write().map_err(|_| Error::PoisonedState)?;
         let new_state = VmState::Shutdown;
@@ -980,12 +984,25 @@ impl Vm {
         let new_state = VmState::Running;
         current_state.valid_transition(new_state)?;
 
+        // On x86_64, load_kernel() invokes configure_system().
+        // But on aarch64, it only loads kernel, configure_system()
+        // need to be postponed after VCPU's are created.
         let entry_addr = self.load_kernel()?;
 
+        // create and configure vcpus
+        self.cpu_manager
+            .lock()
+            .unwrap()
+            .create_boot_vcpus(entry_addr)
+            .map_err(Error::CpuManager)?;
+
+        #[cfg(target_arch = "aarch64")]
+        self.configure_system()?;
+
         self.cpu_manager
             .lock()
             .unwrap()
-            .start_boot_vcpus(entry_addr)
+            .start_boot_vcpus()
             .map_err(Error::CpuManager)?;
 
         if self