Instead of always assuming success for bdrv_aio_flush, actually do something
with the error. This respects the werror option and accordingly ignores the
error, reports it to the guest or stops the VM and retries after cont.

Ignoring the error is trivial, obviously. For stopping the VM and retrying
later old code can be reused, but we need to introduce a new status for "retry
a flush". For reporting to the guest, fortunately the same action is required
as for a failed read/write (status = DRDY | ERR, error = ABRT), so this code
can be reused as well.
Signed-off-by: NKevin Wolf <kwolf@redhat.com>