x86-64, paravirt: Call set_iopl_mask() on 64 bits

set_iopl_mask() is a no-op on 64 bits, but it is also a paravirt hook,
so call it even on 64 bits.

Signed-off-by: H. Peter Anvin <hpa@zytor.com>
Cc: Jeremy Fitzhardinge <jeremy@goop.org>
Cc: Brian Gerst <brgerst@gmail.com>
LKML-Reference: <1260403316-5679-3-git-send-email-brgerst@gmail.com>