assign() raises DeviceAlreadyAssigned exception, not
DeviceAlreadyAttached. While at it, fix reporting changed devices
(report no change if nothing was changed), and also skip re-assigning if
options didn't change.

Stop pretending qubes API uses BDF directly, as now with conversion to
PCI paths it's not plausible anymore. And with a change in grains to
list qubes's API identifiers it's not matching anymore anyway.
But more importantly, using consistent identifiers is important if
device need to have options changed, as the "old" device id need to
match the "new" one - otherwise removing old one from list of assigned
devices (value_combined variable) would fail.

QubesOS/qubes-issues#8681