block: qemu-iotests, add quotes to $TEST_IMG usage in 019

There were still instances of $TEST_IMG not being properly quoted.
This was in the usage of a string built up for a 'for' loop; modify
the loop so we can quote $TEST_IMG properly.

Signed-off-by: Jeff Cody <jcody@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
