hw/m68k/q800: use qemu_find_nic_info()

hw/m68k/q800: use qemu_find_nic_info()

If a corresponding NIC configuration was found, it will have a MAC address
already assigned, so use that. Else, generate and assign a default one.

Using qemu_find_nic_info() is simpler than the alternative of using
qemu_configure_nic_device() and then having to fetch the "mac" property
as a string and convert it.

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