vhost: Code cleaning

After a bug was found, I decided to review old vhost code.
This change cleans some of it by:
- Use unix file private data rather than hash table
- Use vnet interface lookups rather than hash table
- Keep track of unix_fd index rather than fd value
- Change some indent
- Store interfaces in a clib pool rather than hand-made pool

The overall change reduces the number of lines as well as
the amount of state kept in vhost structures.

Change-Id: I6b9b670b9c7e1fb85981bcf2099f7140b0251fe0
Signed-off-by: Pierre Pfister <ppfister@cisco.com>