scripts/kvm/kvm_stat: Cleanup of Stats class

Converted class definition to new style and renamed improper named
variables.

Introduced property for fields_filter.

Moved member variable declaration to init, so one can see all class
variables when reading the init method.

Completely clear the values dict, as we don't need to keep single values.

Signed-off-by: Janosch Frank <frankja@linux.vnet.ibm.com>
Message-Id: <1452525484-32309-23-git-send-email-frankja@linux.vnet.ibm.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
