xen: return xenstore command failures via response instead of rc

xen: return xenstore command failures via response instead of rc

When the xenbus driver does some special handling for a Xenstore
command any error condition related to the command should be returned
via an error response instead of letting the related write operation
fail. Otherwise the user land handler might take wrong decisions
assuming the connection to Xenstore is broken.

While at it try to return the same error values xenstored would
return for those cases.

Signed-off-by: Juergen Gross <jgross@suse.com>
Reviewed-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>
Signed-off-by: Juergen Gross <jgross@suse.com>
OraBug: 25497392

(cherry picked from commit 9a6161fe73bdd3ae4a1e18421b0b20cb7141f680)
Signed-off-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>