KVM: VMX: Move private memory slot position

PCI device assignment would map guest MMIO spaces as separate slot, so it is
possible that the device has more than 2 MMIO spaces and overwrite current
private memslot.

The patch move private memory slot to the top of userspace visible memory slots.

Signed-off-by: Sheng Yang <sheng@linux.intel.com>
Signed-off-by: Avi Kivity <avi@redhat.com>