Intercepting vmbus devices makes VTL2 RAM accessible to VTL0

[cperezvargas]

Make sure we are happy with VtlMemoryProtection::modify_vtl_page_setting and its usage. Alternatives could include setting aside RAM to use for this purpose, changing signature so memory can not be deallocated while not properly protected, always leak memory after this use so it can never be reused, etc.

There's some considerations if we should carve out special regions of the VTL2 GPA ranges so that we cannot use that ram for any other purposes. I think if we know exactly how large this would be, that would be desirable so that we can never have this reuse issue where the we could make pages accessible by VTL0 that VTL2 could later use for something else.

[cperezvargas]

We are still doing this. The issue was opened to see if we can come up with a more creative way of handling this, but since we proposed this we are using it in even more places, so this is not likely something that will change soon.

[chris-oo]

We should probably have intercepted VMBUS devices use private pool/not normal VTL2 memory for these DMA allocations rather than what we're doing today. This might come as part of the DMA refactoring work. However what we have today is functional and correct, just not the cleanest design.