KVM: x86/mmu: Make .write_log_dirty a nested operation

Move .write_log_dirty() into kvm_x86_nested_ops to help differentiate it
from the non-nested dirty log hooks.  And because it's a nested-only
operation.

Signed-off-by: Sean Christopherson <sean.j.christopherson@intel.com>
Message-Id: <20200622215832.22090-5-sean.j.christopherson@intel.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>