KVM: Convert KVM from ->nopage() to ->fault()

Signed-off-by: Nick Piggin <npiggin@suse.de>
Cc: kvm-devel@lists.sourceforge.net
Cc: avi@qumranet.com
Cc: linux-kernel@vger.kernel.org
Signed-off-by: Avi Kivity <avi@qumranet.com>