tools/kvm_stat: don't reset stats when setting PID filter for debugfs

When setting a PID filter in debugfs, we unnecessarily reset the
statistics, although there is no reason to do so. This behavior was
merely introduced with commit 9f114a03 "tools/kvm_stat: add
interactive command 'r'", most likely to mimic the behavior of
the tracepoints provider in this respect. However, there are plenty
of differences between the two providers, so there is no reason not
to take advantage of the possibility to filter by PID without
resetting the statistics.

Signed-off-by: Stefan Raspl <raspl@linux.ibm.com>
Signed-off-by: Radim Krčmář <rkrcmar@redhat.com>