Paging: Only map required areas

[mkroening]

Uhyve identity-maps the first Gibibyte of memory (512 page table entries * 2MiB pages)
We now unmap all memory after the kernel image, so that we can remap it ourselves later for the heap.
Ideally, uhyve would only map as much memory as necessary, but this requires a hermit-entry ABI jump.

https://github.com/hermitcore/libhermit-rs/blob/a86a28e33ffb1655b54d77ff4c301fb24a70b96f/src/arch/x86_64/mm/paging.rs#L219