tools/kvm_stat: add interactive command 'c'

Provide a real simple way to erase any active filter.

Signed-off-by: Stefan Raspl <raspl@linux.vnet.ibm.com>
Reviewed-by: Marc Hartmayer <mhartmay@linux.vnet.ibm.com>
Signed-off-by: Radim Krčmář <rkrcmar@redhat.com>

tools/kvm/kvm_stat/kvm_stat.txt

@@ -29,6 +29,8 @@ meaning of events.
 INTERACTIVE COMMANDS
 --------------------
 [horizontal]
+*c*::	clear filter
+
 *f*::	filter by regular expression
 
 *g*::	filter by guest name