On XEN PV, in order to include reclaimed startup/bootstrap pages, we
need to set low_water to 0, unlike on native.

XEN on amd64  uses the direct map, so include all mapped kernel pages
in the crash dump, just like native.

Approved by: gibbs(implicit)
Update remove comment about resected private allocator.

Approved by:	gibbs (implicit)
Towards single usermode with direct mapped (using 4K pages) XEN PV
kernel (not complete).

We also disable use of the pg_nx bit.

Approved by:	gibbs (implicit)