Rather than just using qemu_configure_nic_device(), populate the MAC
address in the system-registers device by peeking at the NICInfo before
it's assigned to the device.
Generate the MAC address early, if there is no matching -nic option.
Otherwise the MAC address wouldn't be generated until net_client_init1()
runs.
Signed-off-by: David Woodhouse <dwmw@amazon.co.uk> Reviewed-by: Thomas Huth <thuth@redhat.com>