block: qemu-iotests, add quotes to $TEST_IMG.base usage in 017

$TEST_IMG.base is used unquoted. Add quotes so that pathnames with
spaces are supported.
Signed-off-by: NJeff Cody <jcody@redhat.com>
Reviewed-by: NEric Blake <eblake@redhat.com>
Signed-off-by: NStefan Hajnoczi <stefanha@redhat.com>

tests/qemu-iotests/017

@@ -66,7 +66,7 @@ echo "Creating test image with backing file"
 echo
 
 TEST_IMG=$TEST_IMG_SAVE
-_make_test_img -b $TEST_IMG.base 6G
+_make_test_img -b "$TEST_IMG.base" 6G
 
 echo "Filling test image"
 echo