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.

Bonus bug fix: we now check for qdev_init() failing there.

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