Bug fixes and readability improvements to guest memory handling

[hlyytine]

• Fix behaviour (iterator resetting, return value) in map_vm_memory_reservation()
• Avoid potential integer overflows, no matter how theoretical they may be
• Remove duplicate code
• Write complex comparisons in "simple English"

As far as I can see, the only potential side effect of these changes is that mapping done by map_vm_memory_reservation() is no longer allowed to overflow the region. But I can't think of situation where that could anything else than a bug.