Add tests:
- zlib: test_virtio_block_qcow2_zlib()
- zstd: test_virtio_block_qcow2_zstd()
Both these tests use zlib- and zstd-compressed images as OS image.
Modify test_virtio_block_qcow2_backing_file() test: it is practical
to test qcow2 file-backing with compression, so use zlib-compressed
image as a backing file.
Signed-off-by: Eugene Korenevsky <ekorenevsky@aliyun.com>