hw: Simplify calls to pci_nic_init_nofail()

hw: Simplify calls to pci_nic_init_nofail()

pci_nic_init_nofail() calls qemu_find_nic_model(), and this function
sets nd->model = g_strdup(default_model) if it has not been initialized
yet. So we don't have to set nd->model to the default_nic in the
calling sites.

Signed-off-by: Thomas Huth <thuth@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>