[xen] Booting a guest complains about trying to use an already-mounted block device, but it isn't mounted?!

4.11. Booting a guest complains about trying to use an already-mounted block device, but it isn't mounted?!

This is a bug to do with Xen incorrectly freeing guest resources. Take a look at the last line in /var/log/xen-hotplug.log, something like:

xenstore-read: couldn't read path /local/domain/9/vm

Take a note of that number, and run xenstore-rm backend/vbd/9, replacing 9 with the number you read. The guest machine should then boot happily.

Be careful when doing this that there are no guests effectively using the device in question, damage will probably be incurred in that case!