Document the intended usage of each emulation type as each exists to
handle an edge case of one kind or another and can be easily
misinterpreted at first glance.

Cc: Liran Alon <liran.alon@oracle.com>
Signed-off-by: NSean Christopherson <sean.j.christopherson@intel.com>
Signed-off-by: NPaolo Bonzini <pbonzini@redhat.com>