block: mark blocks dirty on coroutine write completion

block: mark blocks dirty on coroutine write completion

The aio write operation marks blocks dirty when the write operation
completes.  The coroutine write operation marks blocks dirty before
issuing the write operation.

It seems safest to mark the block dirty when the operation completes so
that anything tracking dirty blocks will not act before the change has
been made to the image file.

Make the coroutine write operation dirty blocks on write completion.

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