[x86_64] fetch MMIO fault instructions across guest pages safely

[Solicey]

Summary

Fix x86_64 MMIO instruction fetching when a guest instruction crosses a page boundary. (Issue #405)

Problem

The MMIO emulator previously translated guest RIP once, then read 15 consecutive bytes from the resulting host address. Adjacent guest pages are not necessarily adjacent in host memory, so this could read unrelated or unmapped memory.

Changes

• Fetch only the instruction bytes needed by the decoder, translating each guest page independently.
• Limit instruction reads to the x86 maximum of 15 bytes.
• Inject a guest page fault when a required instruction page is not present, preserving RIP and setting guest CR2.
• Preserve guest CR2 across VM entry and exit.
• Pass the MMIO handler its original argument separately from the region base.
• Add focused tests for page-boundary reads and the instruction-length limit.