The decision to branch out to a slower SMT path in instruction
emulation will become a bit more complicated with the way that
"big-core" topology that will be implemented in subsequent changes.
Hide these details from the wider CPU emulation code with a bool
has_smt_siblings flag that can be set by machine initialisation.
Reviewed-by: Cédric Le Goater <clg@redhat.com> Signed-off-by: Nicholas Piggin <npiggin@gmail.com>