net: Fix hotplug with pci_add

net: Fix hotplug with pci_add

The correct model type wasn't getting added when hotplugging nics with
pci_add.

Testcase: start VM with default nic type. In the qemu_monitor:

(qemu) pci_add auto nic model=virtio

This results in a nic hot-plug of the same nic type as the default.

This was broken in 5294e2c774f120e10b44652ac143abda356f44eb

Also changes the behaviour where no .init is defined for a
net_client_type. Previously, 0 was returned, which indicated the init
was successful and that 0 was the index into the nd_tables[] array.
Return -1, indicating unsuccessful init, in such a case.

Signed-off-by: Amit Shah <amit.shah@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>