mem: allow virtual memory address hinting

mem: allow virtual memory address hinting

For multi-process applications, it can sometimes occur that part of the
address ranges used for memory mapping in the primary process are not
free in the secondary process, which causes the secondary processes to
abort on startup.
This patch adds in a memory hinting mechanism, where you can hint a
starting base address to the primary process for where you would like
the hugepage memory to be mapped. It is just a hint, so the memory will
not always go exactly where requested, but it should allow the memory
addresses used by a primary process to be adjusted up or down a little,
thereby fixing issues with secondary process startup.

Signed-off-by: Bruce Richardson <bruce.richardson@intel.com>