KVM: Make shadow pte updates atomic

With guest smp, a second vcpu might see partial updates when the first
vcpu services a page fault.  So delay all updates until we have figured
out what the pte should look like.

Note that on i386, this is still not completely atomic as a 64-bit write
will be split into two on a 32-bit machine.

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