tools/kvm_stat: use a namedtuple for storing the values

Use a namedtuple for storing the values as it allows to access the
fields of a tuple via names. This makes the overall code much easier
to read and to understand. Access by index is still possible as
before.

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