net: Improve the warnings for dubious command line option combinations

net: Improve the warnings for dubious command line option combinations

Improve the warnings we give if the user specified a combination of -net
options which don't make much sense:
 * Don't warn about anything if the config is the implicit default
   "-net user -net nic" rather than one specified by the user (this will
   only kick in for boards with no NIC or if CONFIG_SLIRP is not set)
 * Diagnose the case where the user asked for NICs which the board
   didn't instantiate (for example where the user asked for two NICs
   but the board only supports one)

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Aurelien Jarno <aurelien@aurel32.net>