KVM: MMU: Add for_each_shadow_entry(), a simpler alternative to walk_shadow()

Using a for_each loop style removes the need to write callback and nasty
casts.

Implement the walk_shadow() using the for_each_shadow_entry().

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

diff --cc arch/x86/kvm/mmu.c
Simple merge