pc: s/INT64_MAX/UINT64_MAX/

pc: s/INT64_MAX/UINT64_MAX/

It doesn't make sense for a region to be INT64_MAX in size:
memory core uses UINT64_MAX as a special value meaning
"all 64 bit" this is what was meant here.

While this should never affect the PC system which at the moment always
has < 63 bit size, this makes us hit all kind of corner case bugs with
sub-pages, so users are probably better off if we just use UINT64_MAX
instead.

Reported-by: Luiz Capitulino <[email protected]>
Tested-by: Luiz Capitulino <[email protected]>
Reviewed-by: Michael S. Tsirkin <[email protected]>
Signed-off-by: Michael S. Tsirkin <[email protected]>