Improve export and fix export-on-norun bug

Currently gnt-backup export chains the ShutdownInstance and StartupInstance
opcodes to itself. This works but (a) it's suboptimal, because there's no need
to deactivate the instance's disks as we are about to restart it anyway, and
(b) doesn't take care of instances which are already down (and should be). This
patch takes care of this by just calling the shutdown rpc function instead of
the whole opcode, and just starting up the instance if it's configured as up in
the first place.

Reviewed-by: imsnah