tools/kvm_stat: remove regex filter on empty input

Behavior on empty/0 input for regex and pid filtering was inconsistent, as
the former would keep the current filter, while the latter would (naturally)
remove any pid filtering.
Make things consistent by falling back to the default filter on empty input
for the regex filter dialogue.

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>