The pci_register_device() call in PCI nic initialization routines can
fail.  Handle this failure and propagate a meaningful error message to
the user instead of generating a SEGV.

Cc: Marcelo Tosatti <mtosatti@redhat.com>
Signed-off-by: NChris Wright <chrisw@sous-sol.org>
Signed-off-by: NAvi Kivity <avi@qumranet.com>
Signed-off-by: NAnthony Liguori <aliguori@us.ibm.com>