x86: run-time callers of map_pages_to_xen() must check for errors

x86: run-time callers of map_pages_to_xen() must check for errors

Again, (out-of-memory) errors must not cause hypervisor crashes, and
hence ought to be propagated.

This also adjusts the cache attribute changing loop in
get_page_from_l1e() to not go through an unnecessary iteration. While
this could be considered mere cleanup, it is actually a requirement
for the subsequent now necessary error recovery path.

Also make a few functions static, easing the check for potential
callers needing adjustment.

Signed-off-by: Jan Beulich <jbeulich@novell.com>