libxl: disallow attaching the same device more than once

libxl: disallow attaching the same device more than once

Originally the code allowed users to attach the same device more than
once. It just stupidly overwrites xenstore entries. This is bogus as
frontend will be very confused.

Introduce a helper function to check if the device to be written to
xenstore already exists. A new error code is also introduced.

The check and add are within one xs transaction.

Signed-off-by: Wei Liu <wei.liu2@citrix.com>
Acked-by: Ian Campbell <ian.campbell@citrix.com>