KVM: MMU: Make set_cr3 a function pointer in kvm_mmu

This is necessary to implement Nested Nested Paging. As a
side effect this allows some cleanups in the SVM nested
paging code.

Signed-off-by: Joerg Roedel <joerg.roedel@amd.com>
Signed-off-by: Avi Kivity <avi@redhat.com>