Revert the first part of r259097.
32bit ops on registers automatically clear out the top 32bits of its
64bits.

Suggested by: jilles@
Approved by: gibbs@ (implicit)
Disable the kernel's use of 'xsave' under XEN pv

Approved by:	gibbs@ (implicit)
re-implement pmap_enter_quick_locked()
Do not assume that cleared pte entries are zeroed under XEN.
Test for PG_V instead.

Approved by:	gibbs@ (implicit)