perf stats: fix divide by 0 in variance

Number of samples needs to be greater 1 to have a variance.
Fixes nan% in perf-kvm-live output.

Signed-off-by: David Ahern <dsahern@gmail.com>
perf stats: add max and min stats

Signed-off-by: David Ahern <dsahern@gmail.com>
perf kvm: add min and max stats to results display

Signed-off-by: David Ahern <dsahern@gmail.com>
perf kvm: do not print stats for HLT exits

throws off the display since the times are much larger for HLT
exit reasons.

Signed-off-by: David Ahern <dsahern@gmail.com>
perf kvm: add option to print events that exceed a threshold

Signed-off-by: David Ahern <dsahern@gmail.com>
perf kvm: add debugs if vmexit or vmentry event is missing

Expected to have missing events for each vcpu when perf is
started.

Signed-off-by: David Ahern <dsahern@gmail.com>