cmd/run: Beautify the output when instance is stopped

Before:
   [asias@hjpc capstan]$ capstan run
   Created instance: i1397111888
   OSv v0.06-rc2-31-gdf30ec8

   [/]% [asias@hjpc capstan]$

After:
   [asias@hjpc capstan]$ capstan run
   Created instance: i1397111771
   OSv v0.06-rc2-31-gdf30ec8

   [/]%
   [asias@hjpc capstan]$

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