Instead of writing errors to

Instead of writing errors to
/local/domain/0/backend/<devclass>/<dom>/<devid>/error, write them instead to
/local/domain/0/error/backend/<yada>.  This is not the best place for them
perhaps, but it moves them out of the backend directory, on which the drivers
have a watch.  This fixes the problem whereby writing an error will trigger a
watch, causing the error message to be written again, and repeat.  Fixes bug
#286.

Signed-off-by: Ewan Mellor <ewan@xensource.com>