cmd/run: Make capstan run image_name work

e.g.

   $ capstan run cloudius/osv-iperf
   Created instance: cloudius-osv-iperf
   OSv v0.07
   ------------------------------------------------------------
   Server listening on TCP port 5001
   TCP window size: 64.0 KByte (default)
   ------------------------------------------------------------

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