Mark the default NIC via the new MachineClass->default_nic setting
so that the machine-defaults code in vl.c can decide whether the
default NIC is usable or not (for example when compiling with the
"--without-default-devices" configure switch).

Message-Id: <20230523110435.1375774-3-thuth@redhat.com>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Tested-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Signed-off-by: Thomas Huth <thuth@redhat.com>