Rather than open-coding two different ways to check for an unwanted
negative sign, reuse the same code in both functions.  That way, if we
decide down the road to accept "-0" instead of rejecting it, we have
fewer places to change.  Also, it means we now get ERANGE instead of
EINVAL for negative values in qemu_strtosz, which is reasonable for
what it represents.  This in turn changes the expected output of a
couple of iotests.

The change is not quite complete: negative fractional scaled values
can trip us up.  This will be fixed in a later patch addressing other
issues with fractional scaled values.

Signed-off-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Hanna Czenczek <hreitz@redhat.com>
Message-Id: <20230522190441.64278-18-eblake@redhat.com>