pc: add SMM property

The property can take values on, off or auto.  The default is "off"
for KVM and pre-2.4 machines, otherwise "auto" (which makes it
available on TCG or on new-enough kernels).

Acked-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>