passthrough: release assigned PCI devices earlier during domain shutdown

passthrough: release assigned PCI devices earlier during domain shutdown

At least with xend, where there's not even a tool stack side attempt to
de-assign devices during domain shutdown, this allows immediate re-
starts of a domain to work reliably. (There's no apparent reason why
c/s 18010:c1577f094ae4 chose to put this in the asynchronous part of
domain destruction).

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Acked-by: Keir Fraser <keir@xen.org>