block: switch bdrv_aio_writev() to coroutines More sync, aio, and coroutine unification. Make bdrv_aio_writev() go through coroutine request processing. Remove the dirty block callback mechanism which was needed only for aio processing and can be done more naturally in coroutine context. Signed-off-by: Stefan Hajnoczi <stefanha@linux.vnet.ibm.com> Signed-off-by: Kevin Wolf <kwolf@redhat.com>