paravirt: add hooks for ptep_modify_prot_start/commit

This patch adds paravirt-ops hooks in pv_mmu_ops for ptep_modify_prot_start and
ptep_modify_prot_commit.  This allows the hypervisor-specific backends to
implement these in some more efficient way.

Signed-off-by: Jeremy Fitzhardinge <jeremy.fitzhardinge@citrix.com>
Acked-by: Linus Torvalds <torvalds@linux-foundation.org>
Acked-by: Hugh Dickins <hugh@veritas.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>