tools/kvm_stat: display regex when set to non-default

If a user defines a regex filter through the interactive command, display
the active regex in the header's second line.

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>