qio_channel_yield() now updates ioc->read_write/coroutine and calls
qio_channel_set_aio_fd_handlers(), so the code in the handlers has
become redundant and can be removed.

This does not make a difference in intermediate states because
aio_co_wake() really enters the coroutine immediately here: These
handlers are never run in coroutine context, and we're in the right
AioContext because qio_channel_attach_aio_context() asserts that the
handlers are inactive.

To make these conditions more obvious, assert the right AioContext.

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