xen/pvcalls: check for xenbus_read() errors

Smatch complains that "len" is uninitialized if xenbus_read() fails so
let's add some error handling.

Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Reviewed-by: Juergen Gross <jgross@suse.com>
Reviewed-by: Stefano Stabellini <sstabellini@kernel.org>
Signed-off-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>