net: Consistently use qemu_macaddr_default_if_unset

Drop the open-coded MAC assignment from net_init_nic and replace it with
standard qemu_macaddr_default_if_unset which is also used by qdev. That
avoid creating colliding MACs when instantiating NICs via different
mechanisms.

This change requires to store the MAC as MACAddr in NICInfo, and the
remaining nd_table users need to be updated.

Based on suggestion by Peter Maydell.

CC: Markus Armbruster <armbru@redhat.com>
CC: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>