vmm: Refactor AArch64 GIC initialization process

In the new process, `device::Gic::new()` covers additional actions:
1. Creating `hypervisor::vGic`
2. Initializing interrupt routings

The change makes the vGic device ready in the beginning of
`DeviceManager::create_devices()`. This can unblock the GIC related
devices initialization in the `DeviceManager`.

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

devices/src/interrupt_controller.rs

@@ -55,8 +55,6 @@ pub struct MsiMessage {
 // IOAPIC (X86) or GIC (Arm).
 pub trait InterruptController: Send {
     fn service_irq(&mut self, irq: usize) -> Result<()>;
-    #[cfg(target_arch = "aarch64")]
-    fn enable(&self) -> Result<()>;
     #[cfg(target_arch = "x86_64")]
     fn end_of_interrupt(&mut self, vec: u8);
     fn notifier(&self, irq: usize) -> Option<EventFd>;