tools/kvm_stat: add new command line switch '-i'

It might be handy to display the full history of event stats to compare
the current event distribution against any available historic data.
Since we have that available for debugfs, we offer a respective command
line option to display what's available.

Signed-off-by: Stefan Raspl <raspl@linux.vnet.ibm.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>