x86: fill in missing pv_mmu_ops entries for PAGETABLE_LEVELS >= 3

This finally makes paravirt-ops able to compile and boot under x86_64.

Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>