iotests: Inject space into -ocompat=0.10 in 051

It did not matter before, but now that _make_test_img understands -o, we
should use it properly here.

Signed-off-by: Max Reitz <mreitz@redhat.com>
Reviewed-by: Maxim Levitsky <mlevitsk@redhat.com>
Message-id: 20191107163708.833192-11-mreitz@redhat.com
Signed-off-by: Max Reitz <mreitz@redhat.com>

tests/qemu-iotests/051

@@ -158,7 +158,7 @@ echo
 echo === With version 2 images enabling lazy refcounts must fail ===
 echo
 
-_make_test_img -ocompat=0.10 $size
+_make_test_img -o compat=0.10 $size
 
 run_qemu -drive file="$TEST_IMG",format=qcow2,lazy-refcounts=on
 run_qemu -drive file="$TEST_IMG",format=qcow2,lazy-refcounts=off