[POWERPC][XEN] More Robust Memory Checking

[POWERPC][XEN] More Robust Memory Checking

This patch allows the platform to define the "IO Hole" of
addressibility and checks that even Dom0 does not try to Map "Remote"
memory that is not there.  Also replaces some panic calls with WARN();
and returns failure.

Signed-off-by: Jimi Xenidis <jimix@watson.ibm.com>
Signed-off-by: Hollis Blanchard <hollisb@us.ibm.com>