Fixes for agent to work with un-numbered vhost0

There are two issues in case when vhost0 is unnumbered in host-os.

1. When vhost0 interface is un-numbered in host-os, packets from host-os
   addressed to metadata-ip for VM will use an unknown-ip to vrouter. This
   will result in packets being dropped since source-ip is not-known.
2. When agent runs http-proxy as part of metadata services, packets from
   VM for metadat-service must be NAT-ed to an IP address on host-os.
   When vhost0 is unnumbered, VRouter must use one of the IP address in
   host-os.

Agent is modified to support following,
Agent takes the set of IP address on host-os as vhost_services_ip.
Agent will use one of the vhost_services_ip for NAT-ing of packet from
VM to metadata service