tools: kvm_stat: Add comments

A lot of the code works with the perf events about which only sparse
documentation was available until 2012. Having that information now,
we can clarify what is done in the code.

Signed-off-by: Janosch Frank <frankja@linux.vnet.ibm.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>