[Qemu-arm] [RFC] Add virtual SDEI support in qemu

[Guoheyi]

Hi folks,

Do it make sense to implement virtual SDEI in qemu? So that we can have 
the standard way for guest to handle NMI watchdog, RAS events and 
something else which involves SDEI in a physical ARM64 machine.

My basic idea is like below:

1. Change a few lines of code in kvm to allow unhandled SMC invocations 
(like SDEI) to be sent to qemu, with exit reason of KVM_EXIT_HYPERCALL, 
so we don't need to add new API.

2. qemu handles supported SDEI calls just as the spec says for what a 
hypervisor should do for a guest OS.

3. For interrupts bound to hypervisor, qemu should stop injecting the 
IRQ to guest through KVM, but jump to the registered event handler 
directly, including context saving and restoring. Some interrupts like 
virtual timer are handled by kvm directly, so we may refuse to bind such 
interrupts to SDEI events.

Any comment or suggestion will be really appreciated.

Heyi