hw/net/lasi_i82596: use qemu_create_nic_device()

hw/net/lasi_i82596: use qemu_create_nic_device()

Create the device only if there is a corresponding NIC config for it.
Remove the explicit check on nd_table[0].used from hw/hppa/machine.c
which (since commit d8a3220005d7) tries to do the same thing.

The lasi_82596 support has been disabled since it was first introduced,
since enable_lasi_lan() has always been zero. This allows the user to
enable it by explicitly requesting a NIC model 'lasi_82596' or just
using the alias 'lasi'. Otherwise, it defaults to a PCI NIC as before.

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