xl: Return proper error codes for block-attach and block-detach

xl: Return proper error codes for block-attach and block-detach

Return proper error codes on failure so that scripts can tell whether
the command completed properly or not.

This is not a proper fix, since it fails to call
libxl_device_disk_dispose() on the error path.  But a proper fix
requires some refactoring, so given where we are in the release
process, it's better to have a fix that is simple and obvious, and do
the refactoring once the next development window opens up.

Signed-off-by: George Dunlap <george.dunlap@eu.citrix.com>
Acked-by: Ian Campbell <ian.campbell@citrix.com>