Don't warn about the default network setup that you get if no command line
-net options are specified.  There are two cases that we would otherwise
complain about:

(1) board doesn't support a NIC but the implicit "-net nic" requested one
(2) CONFIG_SLIRP not set, so the implicit "-net nic" sets up a nic that
isn't connected to anything
Signed-off-by: NPeter Maydell <peter.maydell@linaro.org>
Signed-off-by: NAnthony Liguori <aliguori@us.ibm.com>