tools/kvm_stat: indicate dead guests as such

For destroyed guests, kvm_stat essentially freezes with the last data
displayed. This is acceptable for users, in case they want to inspect the
final data. But it looks a bit irritating. Therefore, detect this situation
and display a respective indicator in the header.

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