tools/kvm_stat: fix misc glitches

Addresses
- eliminate extra import
- missing variable initialization
- type redefinition from int to float
- passing of int type argument instead of string
- a couple of PEP8-reported indentation/formatting glitches
- remove unused variable drilldown in class Tui

Signed-off-by: Stefan Raspl <raspl@linux.vnet.ibm.com>
Reviewed-by: Marc Hartmayer <mhartmay@linux.vnet.ibm.com>
Signed-off-by: Radim Krčmář <rkrcmar@redhat.com>