KVM: MMU: Make gfn_to_page() always safe

In case the page is not present in the guest memory map, return a dummy
page the guest can scribble on.

This simplifies error checking in its users.

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