Set up available memory correctly

[IanSeyler]

The E820 (BIOS) and Memory Map (UEFI) need to be parsed correctly instead of, currently, just telling the OS there is 64MiB available.

All memory should be identity mapped starting at 0x0
All available memory should be mapped starting at 0xFFFF800000000000

[IanSeyler]

#80 solves this but may need some further work.