Make it obvious that pci_nic_init() can't fail

Make it obvious that pci_nic_init() can't fail

Before this patch, pci_nic_init() returns NULL when it can't find the
model in pci_nic_models[].  Except this can't happen, because
qemu_check_nic_model_list() just searched for model in
pci_nic_models[], and terminated the program on failure.

Repeating the search here is pointless.  Instead, change
qemu_check_nic_model_list() to return the model's array index.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>