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
Since "qubes: fix event framework", handlers from extensions looks the
same as from the VM class itself, so it isn't possible to order them
correctly. Specification says:
For each class first are called bound handlers (specified in class
definition), then handlers from extensions. Aside from above,
remaining order is undefined.
So, restore this property, which is later correctly used to order
handlers.
Two important fixes are in this commit: handlers from decorators are added when
class is defined (and not when class is instantiated); also multiple events can
be specified in the decorator.
