We already had code to close open host files when reading or writing a
savestate, but due to d35fe1b we also need to close open guest files
when reading a savestate, otherwise DoStateRead fails to delete them.
I was considering an alternative solution where instead of copying and
clearing m_handles, we just set `handle.opened = false;` for each handle
before reading a savestate (but not before writing a savestate).
However, this wouldn't solve the problem of DoStateWriteOrMeasure's
calls to OpenFile failing due to all handles being open. I'm not aware
of any games that have that many handles open, though.