AArch64: Implement AArch64 vCPU states save/restore

This commit adds methods to save/restore AArch64 vCPU registers,
including:

1. The AArch64 `VcpuKvmState` structure.

2. Some `Vcpu` trait methods of the `KvmVcpu` structure to
enable the save/restore of the AArch64 vCPU states.

Signed-off-by: Henry Wang <Henry.Wang@arm.com>

hypervisor/src/lib.rs

@@ -20,6 +20,7 @@
 
 #[macro_use]
 extern crate anyhow;
+#[cfg(target_arch = "x86_64")]
 #[macro_use]
 extern crate log;
 extern crate serde;