tools/kvm_stat: switch to python3

The current shebang does not work in environments that only support python3
and have no python2 installed. Plus there does not seem to be a way to
support python2 and python3 at the same time. Since all known python3 issues
were fixed, and as python3 is the way to go, let's switch over.
Note that the code is still python2 compliant, so folks in bad use can
simply revert the shebang.

Suggested-by: Christian Ehrhardt <christian.ehrhardt@canonical.com>
Signed-off-by: Stefan Raspl <raspl@linux.ibm.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>