iotests: Fix test 156

On one hand, the _make_test_img invocation for creating the target image
was missing a -u because its backing file is not supposed to exist at
that point.

On the other hand, nobody noticed probably because the backing file is
created later on and _cleanup failed to remove it: The quotation marks
were misplaced so bash tried to delete a file literally called
"$TEST_IMG{,.target}..." instead of performing brace expansion. Thus, the
files stayed around after the first run and qemu-img create did not
complain about a missing backing file on any run but the first.

Signed-off-by: Max Reitz <mreitz@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Jeff Cody <jcody@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>

tests/qemu-iotests/156

@@ -38,7 +38,7 @@ status=1	# failure is the default!
 _cleanup()
 {
     _cleanup_qemu
-    rm -f "$TEST_IMG{,.target}{,.backing,.overlay}"
+    rm -f "$TEST_IMG"{,.target}{,.backing,.overlay}
 }
 trap "_cleanup; exit \$status" 0 1 2 3 15
 
@@ -83,7 +83,7 @@ _send_qemu_cmd $QEMU_HANDLE \
     'return'
 
 # Create target image
-TEST_IMG="$TEST_IMG.target.overlay" _make_test_img -b "$TEST_IMG.target" 1M
+TEST_IMG="$TEST_IMG.target.overlay" _make_test_img -u -b "$TEST_IMG.target" 1M
 
 # Mirror snapshot
 _send_qemu_cmd $QEMU_HANDLE \