disk/aio: Cancel AIO thread on cleanup

If the AIO thread is still calling io_getevents() while the exit path
calls io_destroy(), it will segfault. Wait for the thread to finish before
destroying the context.

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>