x86: make certain memory sub-ops return valid values

x86: make certain memory sub-ops return valid values

When a domain's shared info field "max_pfn" is zero,
domain_get_maximum_gpfn() so far returned ULONG_MAX, which
do_memory_op() in turn converted to -1 (i.e. -EPERM). Make the former
always return a sensible number (i.e. zero if the field was zero) and
have the latter no longer truncate return values.

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Acked-by: Tim Deegan <tim@xen.org>