ipfix: Export user specified virtual observation ID

ipfix: Export user specified virtual observation ID

In virtual network, users want more info about the virtual point to observe
the traffic.  It should be a string to provide clear info, not a simple
interger ID.

Introduce "other-config: virtual_obs_id" in IPFIX, which is a string
configured by user.  Introduce an enterprise IPFIX entity
"virtualObsID"(898) to export the value. The entity is a variable-length
string.

Signed-off-by: Wenyu Zhang <wenyuz@vmware.com>
Signed-off-by: Ben Pfaff <blp@ovn.org>