hw/arm: Use MachineClass->default_nic in the sbsa-ref machine

hw/arm: Use MachineClass->default_nic in the sbsa-ref machine

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: <20230524082037.1620952-1-thuth@redhat.com>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Tested-by: Marcin Juszkiewicz <marcin.juszkiewicz@linaro.org>
Signed-off-by: Thomas Huth <thuth@redhat.com>