AArch64: Preparation for vCPU save/restore

This commit ports code from firecracker and refactors the existing
AArch64 code as the preparation for implementing save/restore
AArch64 vCPU, including:

1. Modification of `arm64_core_reg` macro to retrive the index of
arm64 core register and implemention of a helper to determine if
a register is a system register.

2. Move some macros and helpers in `arch` crate to the `hypervisor`
crate.

3. Added related unit tests for above functions and macros.

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

arch/src/aarch64/mod.rs

@@ -42,6 +42,9 @@ pub enum Error {
     /// Error configuring the general purpose registers
     REGSConfiguration(regs::Error),
 
+    /// Error configuring the MPIDR register
+    VcpuRegMPIDR(hypervisor::HypervisorCpuError),
+
     /// Error fetching prefered target
     VcpuArmPreferredTarget(hypervisor::HypervisorVmError),
 
@@ -94,7 +97,7 @@ pub fn configure_vcpu(
         .map_err(Error::REGSConfiguration)?;
     }
 
-    let mpidr = regs::read_mpidr(fd).map_err(Error::REGSConfiguration)?;
+    let mpidr = fd.read_mpidr().map_err(Error::VcpuRegMPIDR)?;
     Ok(mpidr)
 }