Fix crash on vm start with netdevs w/out some info

Some devices, like sr-iov do not have information in the domxml to
represent name and model.

This patch fixes the issue by detecting when we are dealing with a
passed through device and setting special values for name and model.

Change-Id: I5a90fba21075ab2c7ecb0cb75f14ca07f090829e
Signed-off-by: Antoni S. Puimedon <asegurap@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/13540
Reviewed-by: Dan Kenigsberg <danken@redhat.com>