tools/kvm_stat: add hint on '-f help' to man page

The man page update for this new functionality was omitted.

Signed-off-by: Stefan Raspl <raspl@linux.vnet.ibm.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

tools/kvm/kvm_stat/kvm_stat.txt

@@ -88,7 +88,7 @@ OPTIONS
 
 -f<fields>::
 --fields=<fields>::
-	fields to display (regex)
+	fields to display (regex), "-f help" for a list of available events
 
 -h::
 --help::