hw/i386: allow SHPC for Q35 machine

Unmask previously masked SHPC feature in _OSC method.

Signed-off-by: Aleksandr Bezzubikov <zuban32s@gmail.com>
Reviewed-by: Marcel Apfelbaum <marcel@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
(cherry picked from commit a41c78c1)
Signed-off-by: Michael Roth <mdroth@linux.vnet.ibm.com>