qapi/block-core: Add "new" qcow2 options

qcow2 supports more than four options by now, add the new options
(overlap check mode and metadata cache size)

Signed-off-by: Max Reitz <mreitz@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Message-id: 1408557576-14574-5-git-send-email-mreitz@redhat.com
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
