KVM: Lazy guest cr3 switching

KVM: Lazy guest cr3 switching

Switch guest paging context may require us to allocate memory, which
might fail.  Instead of wiring up error paths everywhere, make context
switching lazy and actually do the switch before the next guest entry,
where we can return an error if allocation fails.

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