datapath: Change userspace vport interface to use Netlink attributes.

datapath: Change userspace vport interface to use Netlink attributes.

One of the goals for Open vSwitch is to decouple kernel and userspace
software, so that either one can be upgraded or rolled back independent of
the other.  To do this in full generality, it must be possible to add new
features to the kernel vport layer without changing userspace software.
The customary way to do this in the Linux networking stack is to use
Netlink and in particular Netlink attributes.  This commit adopts that
model for the vport layer.  It does not yet actually start using the
Netlink socket layer, which will come later.

Signed-off-by: Ben Pfaff <blp@nicira.com>
Acked-by: Jesse Gross <jesse@nicira.com>