KVM: MMU: Fix potential race setting upper shadow ptes on nonpae hosts

The direct mapped shadow code (used for real mode and two dimensional paging)
sets upper-level ptes using direct assignment rather than calling
set_shadow_pte().  A nonpae host will split this into two writes, which opens
up a race if another vcpu accesses the same memory area.

Fix by calling set_shadow_pte() instead of assigning directly.

Noticed by Izik Eidus.

Signed-off-by: Avi Kivity <avi@qumranet.com>