tools/kvm_stat: use a more pythonic way to iterate over dictionaries

If it's clear that the values of a dictionary will be used then use
the '.items()' method.

Signed-off-by: Marc Hartmayer <mhartmay@linux.vnet.ibm.com>
Tested-by: Stefan Raspl <raspl@linux.vnet.ibm.com>
[Include fix for logging mode by Stefan Raspl]
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>