KVM: MMU: Use mmu_set_spte() for real-mode shadows

In addition to removing some duplicated code, this also handles the unlikely
case of real-mode code updating a guest page table.  This can happen when
one vcpu (in real mode) touches a second vcpu's (in protected mode) page
tables, or if a vcpu switches to real mode, touches page tables, and switches
back.

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