Documentation: Clarify memory layout

Make sure that it is clear where lower and higher addresses are. And fix
a typo at this chance.

Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>
TODO: VT-d error report was merged

Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>
Documentation: Add description of hypervisor interfaces

This describes the currently available set if interfaces the hypervisor
exposes during runtime to its cells.

Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>