qemu-img/convert: Move bs_n > 1 && -B check down

qemu-img/convert: Move bs_n > 1 && -B check down

It does not make much sense to use a backing image for the target when
you concatenate multiple images (because then there is no correspondence
between the source images' backing files and the target's); but it was
still possible to give one by using -o backing_file=X instead of -B X.

Fix this by moving the check.

(Also, change the error message because -B is not the only way to
 specify the backing file, evidently.)

Cc: qemu-stable <qemu-stable@nongnu.org>
Signed-off-by: Max Reitz <mreitz@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
* applied patch from v1 of series as suggested by author
Signed-off-by: Michael Roth <mdroth@linux.vnet.ibm.com>