KVM: selftests: Add helpers for locally (un)blocking IRQs on x86

Copy KVM-Unit-Tests' x86 helpers for emitting STI and CLI, comments and
all, and use them throughout x86 selftests.  The safe_halt() and sti_nop()
logic in particular benefits from centralized comments, as the behavior
isn't obvious unless the reader is already aware of the STI shadow.

Cc: Manali Shukla <Manali.Shukla@amd.com>
Link: https://lore.kernel.org/r/20241220012617.3513898-1-seanjc@google.com


Signed-off-by: Sean Christopherson <seanjc@google.com>