odp-util: Format tunnel attributes directly from netlink.

When we format most netlink attributes we do so from the netlink
itself, iterating through each one and printing the contents out.
However, for tunnels we don't do this - we first convert to the
OVS userspace representation and then format that. While convienient,
this isn't really ideal as the primary use of printing netlink
attributes is debugging and this conversion is lossy, particularly
when the attributes aren't as expected. The result is that unexpected
keys are silently ignored and the level of detail on errors is
minimal.

This situation becomes worse when we introduce support for Geneve.
The conversion to userspace format requires additional information
which we might not have (ovs-dpctl) and is more complicated than
other attributes so it is likely to be confusing in the event of a
bug. The information from the kernel is self-describing so it's
much more reliable to display it directly from the netlink.

This converts tunnel attribute formatting to be more similar to
other types of attributes. As a nice bonus the output becomes
more compact because it doesn't print zeroed out attributes in
cases where they aren't relevant and therefore not present.

Signed-off-by: Jesse Gross <jesse@nicira.com>
Acked-by: Andy Zhou <azhou@nicira.com>