sled-agent: minor cleanup/refactoring around Disk

[jgallagher]

I was poking around in here to remind myself of how some things worked, and did a little bit of cleanup:

1. Remove the variant field from PooledDisk (this information is now also present in the ZpoolName, so this field had become entirely superfluous)
2. Remove the Disk::is_synthetic() and Disk::is_real() methods. The only callers of these were using them to avoid calling methods that panicked on synthetic disks, so instead, we change those methods to return errors. The callers should handle this fine, and it makes the synthetic disk abstraction a little less leaky.
3. Remove RawDisk::is_real() (no callers).