tools/kvm_stat: simplify the sortkey function

The 'sortkey' function references a value in its enclosing
scope (closure). This is not common practice for a sort key function
so let's replace it. Additionally, the function 'sorted' has already a
parameter for reversing the result therefore the inversion of the
values is unneeded. The check for stats[x][1] is also superfluous as
it's ensured that this value is initialized with 0.

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>