x86: vSMP: Fix is_vsmp_box()

is_vsmp_box() currently does not work on vSMPowered systems,  as pci cfg
space is not read correctly -- This patch fixes it.

Signed-off-by: Ravikiran Thirumalai <kiran@scalex86.org>
Signed-off-by: Ingo Molnar <mingo@elte.hu>