[x86_64] Fetch MMIO fault instructions across guest pages safely #405

When an x86_64 guest triggers an MMIO fault, hvisor translates the guest RIP to a host physical address and reads 15 consecutive bytes from that address for instruction decoding. This assumes that adjacent guest instruction pages are also adjacent in host memory. If the instruction crosses a page boundary, the decoder may read unrelated or unmapped host memory.

The MMIO instruction decoder should fetch only the bytes it needs, translating each guest page independently. If a required guest instruction page is not present, hvisor should inject a guest page fault with the correct fault address and leave RIP unchanged.