AArch64: graceful power-off from external

Hi guys

Got requirement from kata-containers, we want a graceful way to power off the VM from external on aarch64, since it's brutal to use kill -9 at the first place and may leave stale resource behind.

Since x86_64 has provided API action SendCtrlAltDel to gracefully shutdown the microvm from external. I'd like to have the same functionality on AArch64.