x86/paravirt: make set_pte operations common

Remove duplicate set_pte* operations.  PAE still needs to have special
variants of some of these because it can't atomically update a 64-bit
pte, so there's still some duplication.

Signed-off-by: Jeremy Fitzhardinge <jeremy@xensource.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>