multiverse: support verifying the multiverse client device

[richvdh]

Currently the device created by multiverse appears as an unverified device, and there is no way to verify it.

It would be useful to add a way to verify the device. Likely the easiest way would be to allow the user to provide their recovery key (so that we can retrieve the private cross-signing keys from secret storage); an alternative would be to implement interactive verification.

[richvdh]

(This is required for some prototyping work which we are planning)