Unify multi-instance handling in gnt-instance

Start, stop and reboot have the same code for dealing with
multi-instance handling. This patch moves all that into a single generic
function, and leaves only the building of the specific opcode for the
operation in the individual functions.

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Michael Hanselmann <hansmi@google.com>