netdev-linux: Don't treat "system" devices as vports for setting stats.

netdev-linux: Don't treat "system" devices as vports for setting stats.

Linux kernel datapath vports have a "set_stats" method.  Until now,
internal vports have been handled in the userspace netdev library as
type "system", so the "system" netdevs would try to use the vport
"set_stats" method.  Now, however, internal netdevs have been broken out
as a separate netdev type, so only that new type of netdev has to be able
to call into "set_stats".  This commit, therefore, removes it from the
"system" netdevs.