tools/kvm_stat: re-animate display of dead guests

When filtering by guest (interactive commands 'p'/'g'), and the respective
guest was destroyed, detect when the guest is up again through the guest
name if possible.
I.e. when displaying events for a specific guest, it is not necessary
anymore to restart kvm_stat in case the guest is restarted.

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