Allow omdb to work with different disk types

[askfongjojo]

With the disk type schema change, omdb db disks no longer understands how to get the disk info:

root@oxz_switch0:~# omdb db disks info fff17edb-b345-42de-9a16-8f52fc6d42ea
Error: Object (of type ById(fff17edb-b345-42de-9a16-8f52fc6d42ea)) not found: disk

[jmpesp]

Error: Object (of type ById($UUID)) not found: disk results from

        let (.., disk) =                                                                                                                    
            LookupPath::new(opctx, self).disk_id(disk_id).fetch().await?;

at the beginning of DataStore::disk_get, which is checking omdb's opctx, which doesn't have the required grants to access all disks. Other disk commands query the database directly.