disk/loopback: Support transparent decompression of backing file

disk/loopback: Support transparent decompression of backing file

A new option is added to the loopback command, -D or --decompress, which
when specified transparently decompresses the backing file. This allows
compressed images to be used as if they were uncompressed.

Add documentation to support this change.

Suggested-by: Li Gen <ligenlive@gmail.com>
Signed-off-by: Glenn Washburn <development@efficientek.com>
Reviewed-by: Daniel Kiper <daniel.kiper@oracle.com>