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 c469117d ("tools/kvm_stat:
simplify initializers").

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