instance: Initial support

Add 'capstan instances' command to list all the instances started by
capstan. It looks like:

$ capstan instances
Name                 Platform   Status     Image
i1396582979          qemu       Running
i1396582986          qemu       Running
i1396582636          vmw        Running
i1396582975          vmw        Running

Signed-off-by: Asias He <asias@cloudius-systems.com>
Merge pull request #56 from asias/instance

Instance