disk/aio: Fix AIO thread

Currently when the kernel completes a batch of AIO requests and signals it
via eventfd, we retrieve at most AIO_MAX events (256), and ignore the
rest. Call io_getevents() again in case more events are pending.

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>