First, cache objects created with init_volume - this is the only place
where we have full volume configuration (including snap_on_start and
save_on_stop properties).
But also implement get_volume method, to get a volume instance for given
volume id. Such volume instance may be incomplete (other attributes are
available only in owning domain configuration), but it will be enough
for basic operations - like cheching and changing its size, cloning
etc.
Listing volumes still use list of physically present volumes.
This makes it possible to start qubesd service, without physical
presence of some storage devices. Starting VMs using such storage would
still fail, of course.
FixesQubesOS/qubes-issues#2960
6e5fe58128