tools/kvm_stat: fix trace setup glitch on field updates in TracepointProvider

Updating the fields of the TracepointProvider does not propagate changes to the
tracepoints. This shows when a pid filter is enabled, whereby subsequent
extensions of the fields of the Tracepoint provider (e.g. by toggling
drilldown) will not modify the tracepoints as required.
To reproduce, select a specific process via interactive command 'p', and
enable drilldown via 'x' - none of the fields with the braces will appear
although they should.
The fix will always leave all available fields in the TracepointProvider
enabled.

Signed-off-by: Stefan Raspl <raspl@linux.vnet.ibm.com>
Based-on-text-by: Janosch Frank <frankja@linux.vnet.ibm.com>
Signed-off-by: Radim Krčmář <rkrcmar@redhat.com>