libxl/devd: correctly manipulate the dguest list

libxl/devd: correctly manipulate the dguest list

Current code in backend_watch_callback has two issues when manipulating the
dguest list:

1. backend_watch_callback forgets to remove a libxl__ddomain_guest from the
list of tracked domains when the related data is freed, causing dereferences
later on when the list is traversed. Make sure that a domain is always removed
from the list when freed.

2. A spurious device state change can cause a dguest to be freed, with active
devices and without being removed from the list. Fix this by always checking if
a dguest has active devices before freeing and removing it.

Signed-off-by: Roger Pau Monné <roger.pau@citrix.com>
Reported-by: Reinis Martinsons <admin@frp.lv>
Suggested-by: Ian Jackson <ian.jackson@eu.citrix.com>
Reviewed-by: Wei Liu <wei.liu2@citrix.com>
Acked-by: Ian Jackson <ian.jackson@eu.citrix.com>
Release-acked-by: Julien Grall <julien.grall@arm.com>