tools/kvm_stat: Fix python3 syntax

tools/kvm_stat: Fix python3 syntax

BugLink: http://bugs.launchpad.net/bugs/1734130
$ python3 tools/kvm/kvm_stat/kvm_stat
  File "tools/kvm/kvm_stat/kvm_stat", line 1137
    def sortkey((_k, v)):
                ^
SyntaxError: invalid syntax

Fix it in a way that's compatible with python2 and python3

Signed-off-by: Cole Robinson <crobinso@redhat.com>
Tested-by: Stefan Raspl <stefan.raspl@linux.vnet.ibm.com>
Signed-off-by: Radim Krčmář <rkrcmar@redhat.com>
(cherry picked from commit 6ade1ae84c9236cbb58de4b17f770f2791106676 linux-next)
Signed-off-by: Seth Forshee <seth.forshee@canonical.com>
Acked-by: Thadeu Lima de Souza Cascardo <cascardo@canonical.com>
Signed-off-by: Thadeu Lima de Souza Cascardo <cascardo@canonical.com>