tools/kvm_stat: fix updates for dead guests

With pid filtering active, when a guest is removed e.g. via virsh shutdown,
successive updates produce garbage.
Therefore, we add code to detect this case and prevent further body updates.
Note that when displaying the help dialog via 'h' in this case, once we exit
we're stuck with the 'Collecting data...' message till we remove the filter.

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