split guest/context switching from hmain.c to context.c and it also replaces switching.c
1. guest context switch now allows trap handler to run the rest of the code until eret
   (previously, __mon_switch_to_next_guext_context() did not allow this)
2. interrupt test code now demonstrates simple guest context switching in an interrupt handler (or ISR)
3. bmguest/ now iterates it's internal loop 300 times and doesn't request for hvc_yield() to show the periodic context switching works
Merge branch 'proto2' of https://github.com/in1004kyu/khypervisor into proto2