Pass Instance object to compute vnc rpc api

Pass a real Instance object to the get_vnc_console() method of the
compute rpc api.  The object_compat() decorator is used to provide
backwards compatibility with receiving data in the old format.

Change-Id: Ib615fd653e0ef6196241fde81f541b1e18eb3ced
Merge "Pass Instance object to compute vnc rpc api"