vhost: vhost-user component may become unusable with too many open files (VPP-668)

vhost: vhost-user component may become unusable with too many open files (VPP-668)

When the number of open files is reached in the system, vhost may
encounter a failure in socket call and return from vhost-user-process.
The return terminates all attempts of incoming socket connections
in the future, even if the condition is reconciled.

The fix is to not return from vhost-user-process, record the error in
the interface, spit out the error, and retry the connection every 3 seconds.

Change-Id: I806baedf13e8c9b73e7c7820c094240f39949950
Signed-off-by: Steven <sluong@cisco.com>