Recover naive VMO implementation

[wangrunji0408]

To achieve efficient copy-on-write, current VMOPaged implementation uses an ingenious data structure, which introduced complexity and endless bugs.

For example, an early version(e0ea3b0) can steadily run Zircon to shell in libos. But now it will trigger SIGFAULT or SIGBUS (#52). Even on bare-metal, sometimes there are mysterious memory errors. These bugs are difficult to fix and impact the functionality.

So for simplicity and stability, I'd like to restore the historical version of VMOPaged. Current version will not be affected and we will continue to maintain them.