block/raw-win32: implement .bdrv_detach/attach_aio_context()

block/raw-win32: implement .bdrv_detach/attach_aio_context()

Drop the assumption that we're using the main AioContext for raw-win32.
Convert the aio-win32 code to support detach/attach and replace
qemu_aio_wait() with aio_poll().

The .bdrv_detach/attach_aio_context() interfaces move the aio-win32
event notifier from the old to the new AioContext.

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