Fix pci_add nic not to exit on bad model

Fix pci_add nic not to exit on bad model

Monitor command "pci_add ADDR nic model=MODEL" uses pci_nic_init() to
create the NIC.  When MODEL is unknown or "?", this prints to stderr
and terminates the program.

Change pci_nic_init() not to treat "?" specially, and to return NULL
on failure.  Switch uses during startup to new convenience wrapper
pci_nic_init_nofail(), which behaves just like pci_nic_init() used to
do.

[markmc:

  - rebase to stable-0.11
  - drop qemu_error() usage
  - go back to pci_create() in pci_nic_init
  - qdev_init() doesn't have an error return
]

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Signed-off-by: Mark McLoughlin <markmc@redhat.com>
Fedora-patch: qemu-do-not-exit-on-pci-hotplug-invalid-nic2.patch