Implement required event handlers according to documentation in
qubes.devices.
A modification of qubes.devices.DeviceInfo is needed to allow dynamic,
read-only properties.
QubesOS/qubes-issues#2257
Add 'backenddomain' element when source (not target) domain is not dom0.
Fix XML elemenet name. Actually set volume.domain when listing
VM-exposed devices.
QubesOS/qubes-issues#2256
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