x86: always set up SMP

Currently setup_vm cannot assume that it can invoke IPIs, and therefore
only initializes CR0/CR3/CR4 on the CPU it runs on.  In order to keep the
initialization code clean, let's just call smp_init (and therefore
setup_idt) unconditionally.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>