qcow2: Add iotest for an image with header.refcount_table_offset == 0

This patch adds a simple iotest in which we try to write to an image
with the refcount table offset set to 0.

This scenario was already handled by the existing consistency checks,
but we add an explicit test case for completeness.
Signed-off-by: NAlberto Garcia <berto@igalia.com>
Message-id: feeceada92486bb8790b90f303fc9fe82a27391a.1509718618.git.berto@igalia.com
Reviewed-by: NMax Reitz <mreitz@redhat.com>
Signed-off-by: NMax Reitz <mreitz@redhat.com>

tests/qemu-iotests/060

@@ -277,6 +277,13 @@ _make_test_img 64M
 poke_file "$TEST_IMG" "56"                "\x00\x00\x00\x00"
 $QEMU_IO -c "write 0 64k" "$TEST_IMG" 2>&1 | _filter_testdir | _filter_imgfmt
 
+echo
+echo "=== Testing incorrect refcount table offset ==="
+echo
+_make_test_img 64M
+poke_file "$TEST_IMG" "48"                "\x00\x00\x00\x00\x00\x00\x00\x00"
+$QEMU_IO -c "write 0 64k" "$TEST_IMG" | _filter_qemu_io
+
 # success, all done
 echo "*** done"
 rm -f $seq.full

tests/qemu-iotests/060.out

@@ -208,4 +208,10 @@ write failed: Input/output error
 
 Formatting 'TEST_DIR/t.IMGFMT', fmt=IMGFMT size=67108864
 can't open device TEST_DIR/t.IMGFMT: Image does not contain a reference count table
+
+=== Testing incorrect refcount table offset ===
+
+Formatting 'TEST_DIR/t.IMGFMT', fmt=IMGFMT size=67108864
+qcow2: Marking image as corrupt: Preventing invalid allocation of L2 table at offset 0; further corruption events will be suppressed
+write failed: Input/output error
 *** done