qemu-iotests: Quote $TEST_IMG* and $TEST_DIR usage

A lot of image filename and paths are used unquoted.  Quote these to
make sure that directories / filenames with spaces are not problematic.

Signed-off-by: Jeff Cody <jcody@redhat.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
diff --git a/tests/qemu-iotests/021 b/tests/qemu-iotests/021
index 6da79eb..1c69024 100755
--- a/tests/qemu-iotests/021
+++ b/tests/qemu-iotests/021
@@ -53,7 +53,7 @@
     for op in $TEST_OPS; do
         echo
 	echo "== testing $op -P $pattern =="
-	$QEMU_IO -c "$op -P $pattern 0 4096" $TEST_IMG | _filter_qemu_io
+	$QEMU_IO -c "$op -P $pattern 0 4096" "$TEST_IMG" | _filter_qemu_io
     done
 done