posix-aio: merge posix_aio_process_queue and posix_aio_read

posix_aio_read already calls qemu_aio_process_queue, and dually
qemu_aio_process_queue is always followed by a select loop that calls
posix_aio_read.

No races are possible, so there is no need for a separate process_queue
callback.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
