linux-aio: use LinuxAioState from the running thread

linux-aio: use LinuxAioState from the running thread

Remove usage of aio_context_acquire by always submitting asynchronous
AIO to the current thread's LinuxAioState.

In order to prevent mistakes from the caller side, avoid passing LinuxAioState
in laio_io_{plug/unplug} and laio_co_submit, and document the functions
to make clear that they work in the current thread's AioContext.

Signed-off-by: Emanuele Giuseppe Esposito <eesposit@redhat.com>
Message-Id: <20230203131731.851116-2-eesposit@redhat.com>
Reviewed-by: Kevin Wolf <kwolf@redhat.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>