block: add bdrv_get_aio_context()

For now bdrv_get_aio_context() is just a stub that calls
qemu_aio_get_context() since the block layer is currently tied to the
main loop AioContext.

Add the stub now so that the block layer can begin accessing its
AioContext.

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