kvmexit.py: introduce a tool to show kvm exit reasons and counts

kvmexit.py: introduce a tool to show kvm exit reasons and counts

Considering virtual machines' frequent exits can cause performance
problems, introduce a tool to show kvm exit reasons and counts, so
that the most frequent exited reasons could be located, reduced, or
even avoided.

For better performance, this tool employs a percpu array and percpu
hash in bpf to store exit reason and its counts. Besides, the bcc
python provides aggregation and various custom output. For more
background, realization and examples, please see kvmexit_example.txt
and man/man8/kvmexit.8 for more reference.

Signed-off-by: Fei Li <lifei.shirley@bytedance.com>