bhyve: rework BAR emulation

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Summary

If the guest writes ~0U to one register of a 64 bit BAR (to read it's size), bhyve registers a BAR with an invalid address. This patch avoids this behaviour.

Additionally, this BAR emulation is required for GVT-d (see D26209) and for GPU Passthrough of AMD graphic cards (see D27456).

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This issue is resolved by D31819.