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 <dgilbert@redhat.com>
Cc: Eduardo Habkost <ehabkost@redhat.com> (maintainer:X86)
Cc: Kevin Wolf <kwolf@redhat.com> (supporter:Block layer core)
Cc: Max Reitz <mreitz@redhat.com> (supporter:Block layer core)
Cc: qemu-block@nongnu.org (open list:Block layer core)
Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
Message-Id: <1487708048-2131-23-git-send-email-armbru@redhat.com>
