x86: include/asm-x86/paravirt.h: x86_64 mmu operations

Add .set_pgd field to pv_mmu_ops.

Implement pud_val(), __pud(), set_pgd(), pud_clear(), pgd_clear().

pud_clear() and pgd_clear() are implemented simply using set_pud()
and set_pmd(). They don't have a field at pv_mmu_ops.

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