KVM: x86: Refactor can_emulate_instruction() return to be more expressive

Refactor and rename can_emulate_instruction() to allow vendor code to
return more than true/false, e.g. to explicitly differentiate between
"retry", "fault", and "unhandleable".  For now, just do the plumbing, a
future patch will expand SVM's implementation to signal outright failure
if KVM attempts EMULTYPE_SKIP on an SEV guest.

No functional change intended (or rather, none that are visible to the
guest or userspace).

Link: https://lore.kernel.org/r/20230825013621.2845700-4-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>