aio-posix: Assert that aio_poll() is always called in home thread

aio-posix: Assert that aio_poll() is always called in home thread

aio_poll() has an existing assertion that the function is only called
from the AioContext's home thread if blocking is allowed.

This is not enough, some handlers make assumptions about the thread they
run in. Extend the assertion to non-blocking calls, too.

Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>