qapi/string-output-visitor: fix bugs

qapi/string-output-visitor: fix bugs

in human mode, we are creating the string:

16-31 (16-31)

instead of

16-17 (10-1f)

because we forgot to pass 'true' as the human parameter on one of the
two calls to format_string.
Also, this is a worsening of quality; previously we would produce

16 (0x10)

to make it obvious which number was hex.
Fix these issues.

Reported-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>