util/cutils: Return qemu_strtosz*() error and value separately

util/cutils: Return qemu_strtosz*() error and value separately

This makes qemu_strtosz(), qemu_strtosz_mebi() and
qemu_strtosz_metric() similar to qemu_strtoi64(), except negative
values are rejected.

Cc: Dr. David Alan Gilbert <[email protected]>
Cc: Eduardo Habkost <[email protected]> (maintainer:X86)
Cc: Kevin Wolf <[email protected]> (supporter:Block layer core)
Cc: Max Reitz <[email protected]> (supporter:Block layer core)
Cc: [email protected] (open list:Block layer core)
Signed-off-by: Markus Armbruster <[email protected]>
Reviewed-by: Eric Blake <[email protected]>
Reviewed-by: Dr. David Alan Gilbert <[email protected]>
Message-Id: <1487708048[email protected]>