bhyve/aarch64: Handle unknown exceptions

[andrew]

Summary

When an exception class is unhandled by the kernel we handle it in
userspace by exiting the process. Rather than exiting raise an unknown
reason exception in the guest. The guest can then handle the exception
as it wishes.

Sponsored by: Arm Ltd

[emaste]

Are there any security implications (i.e., exceptions that are not safe to avoid handling)?

[andrew]

I can't think of any issues. By my reading of Linux they do the same, although handle it in the kernel.