xen/privcmd: Fix mmap batch ioctl error status copy back.

Copy back of per-slot error codes is only necessary for V2. V1 does not provide
an error array, so copyback will unconditionally set the global rc to EFAULT.
Only copyback for V2.

Signed-off-by: Andres Lagar-Cavilla <andres@lagarcavilla.org>
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>