pc: memhp: force gaps between DIMM's GPA

pc: memhp: force gaps between DIMM's GPA

mapping DIMMs non contiguously allows to workaround
virtio bug reported earlier:
http://lists.nongnu.org/archive/html/qemu-devel/2015-08/msg00522.html
in this case guest kernel doesn't allocate buffers
that can cross DIMM boundary keeping each buffer
local to a DIMM.

Suggested-by: Michael S. Tsirkin <[email protected]>
Signed-off-by: Igor Mammedov <[email protected]>
Acked-by: Eduardo Habkost <[email protected]>
Reviewed-by: Michael S. Tsirkin <[email protected]>
Signed-off-by: Michael S. Tsirkin <[email protected]>