Fix check in gnt-instance modify -t

Currently when changing the instance disks the instance status (which
must be down) is only checked if the target status is among the mirrored
ones. This is incorrect: the instance must be down either way, even if
the desired final status is non-mirrored.

Signed-off-by: Guido Trotter <ultrotter@google.com>
Reviewed-by: Iustin Pop <iustin@google.com>