disk/aio: Refactor AIO code

Move all AIO code to a separate file, disk/aio.c, to remove as much
#ifdefs as possible. Split the raw read/write disk ops into async and
sync, and choose which ones to use depending on CONFIG_HAS_AIO. Note that
we fix raw_image__close() which incorrectly checked CONFIG_HAS_VIRTIO
instead of CONFIG_HAS_AIO, and closed an unitialized disk->evt. A
subsequent commit will complete this refactoring by fixing use of the
'async' disk attribute.

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>