microvm: missing cbit in virtio-net-pci when using sev/snp confidential vms on amd

When using https://github.com/project-oak (confidential vms on amd sev-snp). I need to apply this patch : https://github.com/dingelish/qemu/commit/876e262744a6483a3bcb37a07767e6c75f9cf4ee.patch to make it work properly (and it works very well).

There is a bit more context in the following issue https://github.com/project-oak/oak/issues/5044

I'm not sure about what would be the proper way of getting that fixed so I don't need the patch anymore.

edit: the root cause could be that the microvm puts the 64bits pci mmio hole without accounting the addressable space reduction induced by the cbit. And since the guest is stripping that bit, the lookup was failing. I've added what could be a proper patch. The issue only arises for microvm, it works for q35 ones.

Thanks !