This will allow starting processes and calling RPC services in those
events. This if required for usb devices, which are attached using RPC
services.
Intentionally keep device listing events synchronous only - to
discourage putting long-running actions there.
This change also require some not-async attach method version for
loading devices from qubes.xml - have `load_persistent` for this.
When device extension do not return some "persistent" device as
currently attached, still return it, as it will be attached at next
domain startup. User can distinguish such devices by having
frontend_domain=None (or other VM).
Also, return a set from DeviceCollection.assignments().
Allow device plugin to list attached and available devices. Enforce
at API level every device being exposed by some domain.
This commit only changes devices API, but not update existing users
(pci) yet.
QubesOS/qubes-issues#2257
