ivshmem-plain cannot be used on non-Linux hosts
Additional information:
I would like to propose this patch as-is on the mailing list (the trivial one?) as soon as I figure patch submission out fully:

https://github.com/fredldotme/qemu/commit/e929b8db8078aede6df7b02d8c0b71d1e2d6afcb

It's just `#ifdef`ing out doorbell support on non-Linux builds which seems to be enough for basic functionality.