tools/kvm_stat: fix missing field update after filter change

When updating the fields filter, tracepoint events of fields previously
not visible were not enabled, as TracepointProvider.update_fields()
updated the member variable directly instead of using the setter, which
triggers the event enable/disable.
To reproduce, run 'kvm_stat -f kvm_exit', press 'c' to remove the
filter, and notice that no add'l fields that do not match the regex
'kvm_exit' will appear.
This issue was introduced by commit c469117df059 ("tools/kvm_stat:
simplify initializers").

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

tools/kvm/kvm_stat/kvm_stat

@@ -549,8 +549,8 @@ class TracepointProvider(Provider):
 
     def update_fields(self, fields_filter):
         """Refresh fields, applying fields_filter"""
-        self._fields = [field for field in self.get_available_fields()
-                        if self.is_field_wanted(fields_filter, field)]
+        self.fields = [field for field in self.get_available_fields()
+                       if self.is_field_wanted(fields_filter, field)]
 
     @staticmethod
     def get_online_cpus():