tools/kvm_stat: handle guest removals more gracefully

When running with the DebugFS provider, removal of a guest can result in a
negative CurAvg/s, which looks rather confusing.
If so, suppress the body refresh and print a message instead.
To reproduce, have at least one guest A completely booted. Then start
another guest B (which generates a huge amount of events), then destroy B.
On the next refresh, kvm_stat should display a whole lot of negative values
in the CurAvg/s column.

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