1. Do not detach device forcefully when it's removed. This breaks
libvirt (which thinks the device is still there). After this change, it
is possible to detach device using libvirt, even if it was already
removed physically from backend domain (unless it is dom0 - in which
case it is still broken). So, this is partial fix for
QubesOS/qubes-issues#1082.
2. Do not trigger "change" udev event when only QubesDB state needs to
be updated - this leads to massive udev events queue, and heavy I/O
usage - for example scanning all LVM many times. In some cases it even
caused infinite event queue.
3. Do not use QUBES_EXPOSED udev property - it was needed a while back
before QubesDB, because concurrent xenstore accesses are expensive
(because of transactions). It isn't the problem on QubesDB.
4. Cache information about device-mapper, so it is possible to
reconstruct it at device remove - when the actual device cannot be
queried anymore. This is specifically about list of lower layer devices
used.
5. Allow excluding loop devices pointing at a file in directory marked
with ".qubes-exclude-block-devices" file. This is more generic than
hardcoding /var/lib/qubes.
QubesOS/qubes-issues#3084FixesQubesOS/qubes-issues#3073QubesOS/qubes-issues#1082
