Sim sled-agent: Stopped instances lose their disks

The sim sled-agent now marks disks as detached when an instance is in the stopped state because of the fix for https://github.com/oxidecomputer/omicron/issues/2675 ( https://github.com/oxidecomputer/omicron/commit/207768ea2bdc644da2c5c7b5f7ababab4a6f4e95). An instance in stopped state should probably still own the disks (although they are not mounted/attached physically). If not, when the instance is started again, it will have no disk.

@smklein, @jmpesp - The change is only confined to the simulated sled-agent. Does it reflect how a real sled would work in the stopped and destroyed instance scenarios? (To be clear, I'm less concerned if this is only a sim sled-agent issue and won't happen to a real sled.)