specs/qcow2: Clarify that compressed clusters have the COPIED bit reset

Compressed clusters are not supposed to have the COPIED bit set, but
this is not made explicit in the specs, so let's document it.

Signed-off-by: Alberto Garcia <berto@igalia.com>
Message-id: 74552e1d6e858d3159cb0c0e188e80bc9248e337.1523376013.git.berto@igalia.com
Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Max Reitz <mreitz@redhat.com>