disks: want to be able to differentiate "kinds"

all NVMe disks are not created equal: the characteristics of disks backed by Crucible, a file, in-memory, 9pfs, etc, all vary wildly. in the product, the only disk types (currently) are the typical Crucible-backed distributed disks and "local" volumes that are from Propolis' perspective a file-backed disk with a particular file that has desirable performance properties. a VM can have one or all of these things at the same time!

it would be nice as a guest to tell which disks you have. that is, it would be nice for nvme list in a Linux guest to tell you enough that you should put your critical data on the Crucible volume mounted at /dev/nvme2n1 and your more ephemeral information on the local volume at /dev/nvme1n1. that might be an NVMe model number, that might be a PCI device ID, could be both!

since Omicron is really what knows what a local volume is, rather than pointing Propolis at any other random file on a sled, it seems like Nexus ought to set an informative model number, but it probably makes sense for Propolis to own the device ID selection. it would be really weird for propolis-api to let you specify whatever random PCI device ID for a disk, but that comes at the cost of Propolis not really knowing if the file backing a disk is a magic fast volume "file" or a normal file on a ZFS filesystem. the latter isn't something we'll use in the product, so that might be an OK ambiguity.

could have sworn we had an issue for this, but I couldn't find one!