disk/aio: Fix use of disk->async

Add an 'async' attribute to disk_image_operations, that describes if they
can submit async I/O or not. disk_image->async is now set iff
CONFIG_HAS_AIO and the ops do use AIO.

This fixes qcow1, which used to set async = 1 even though the qcow
operations don't use AIO. The disk core would perform the read/write
operation without pushing the completion onto the virtio queue, and the
guest would be stuck waiting.

Reviewed-by: Andre Przywara <andre.przywara@arm.com>
Signed-off-by: Jean-Philippe Brucker <jean-philippe.brucker@arm.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>