tools/kvm_stat: fix extra handling of 'help' with fields filter

Commit 67fbcd62 ("tools/kvm_stat: add '-f help' to get the available
event list") added support for '-f help'. However, the extra handling of
'help' will also take effect when 'help' is specified as a regex in
interactive mode via 'f'. This results in display of all events while
only those matching this regex should be shown.

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