xen: limit extra memory to a certain ratio of base

If extra memory is very much larger than the base memory size
then all of the base memory can be filled with structures reserved to
describe the extra memory, leaving no space for anything else.

Even at the maximum ratio there will be little space for anything else,
but this change is intended to at least allow the system to boot rather
than crash mysteriously.

Signed-off-by: Jeremy Fitzhardinge <jeremy.fitzhardinge@citrix.com>