main-loop: add qemu_get_aio_context()

It is very useful to get the main loop AioContext, which is a static
variable in main-loop.c.

I'm not sure whether qemu_get_aio_context() will be necessary in the
future once devices focus on using their own AioContext instead of the
main loop AioContext, but for now it allows us to refactor code to
support multiple AioContext while actually passing the main loop
AioContext.

Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Reviewed-by: Paolo Bonzini <pbonzini@redhat.com>
