docs: add dirty bit to qcow2 specification

The dirty bit will make it possible to perform lazy refcount updates,
where the image file is not kept consistent all the time.  Upon opening
a dirty image file, it is necessary to perform a consistency check and
repair any incorrect refcounts.

Therefore the dirty bit must be an incompatible feature bit.  We don't
want old programs accessing a file with stale refcounts.

Signed-off-by: Stefan Hajnoczi <stefanha@linux.vnet.ibm.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
