Instance Creation should allow concurrent device creation

Currently, the act of merely "creating" and instance triggers it to begin running as well. In a fully device-less world, this has functioned as a convenience, but within the context of attaching disks and network interfaces, this becomes more of a pain.

In a world without hot-plug, consider the following:

• You want to create an instance
• You want to attach a disk to it

If you call "project_create_instance", the instance is created, immediately transitions to running, and must be stopped before a disk can be attached.

Instead, I propose that instances are initialized with the "Stopped" state, and only become allocated to a particular sled once explicitly started via instance_start. This will provide a created-but-not-started window in which devices may be attached.