route: Add host route support

Add similar function of the following route command
in Freebsd to OSv.

   $ route add -host $host_ip -interface $interface_name
   e.g.
   $ route add -host 10.240.0.1 -interface eth0

This is useful when the gateway is not reachable within the network
specified by the interface. For instance, GCE's DHCP server provides:

   ip=10.240.10.15, netmask=255.255.255.255, gw=10.240.10.1

A host route entry is needed before we can add the default route entry.

With this patch, the route table looks like:

   Destination         Gateway             Flags  Netif
   default             10.240.0.1          UGS    eth0
   10.240.0.1          42:01:0a:f0:e0:c5   UHS    eth0
   10.240.10.15        link#2              U      eth0
   10.240.10.15        link#2              UHS    lo0
   127.0.0.1           link#1              UH     lo0

Signed-off-by: Asias He <asias@cloudius-systems.com>
Signed-off-by: Avi Kivity <avi@cloudius-systems.com>