netdev-vport: Use dpif_port as base for tunnel backing port.

In most cases, tunnel ports specify a dpif name to act as the backing
port in the datapath. However, in the case of UDP tunnels the type is
used with the port number appended. This is potentially a problem for
IPsec tunnels because they have different types but should have the
same backing port. The hasn't been a problem in practice though because
no UDP tunnels are currently used with IPsec.

This switches to use the dpif_port in all cases plus a port number if
necessary. It does this by making the names short enough to accomodate
ports, which also makes the naming more consistent.

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