hw/arm/npcm7xx: use qemu_configure_nic_device, allow emc0/emc1 as aliases

Also update the test to specify which device to attach the test socket
to, and remove the comment lamenting the fact that we can't do so.

Signed-off-by: David Woodhouse <dwmw@amazon.co.uk>
Reviewed-by: Thomas Huth <thuth@redhat.com>