Register privacy for interrupts

Enable clearing for context switches and interrupts. For now register contents can be snooped be snooped across box boundaries, by triggering own box interrupts while executing other boxes threads.

Start by locating the code that handles context switches and interrupts in uVisor. Determine how register contents cross box boundaries today, then verify that clearing occurs for both context switches and interrupts so one box cannot snoop another box's thread registers.

Written by the indexing model from the issue text.