xenbaked.c: Avoid divide by zero issue

xenbaked.c: Avoid divide by zero issue

xenbaked.c -> dump_stats(), run_time = time(&end_time) - time(&start_time),
time() returns the value in seconds. If one cancels xenmon.py immediately
after started, run_time can be zero, and then xenbaked will hit divide by
zero fault.

Signed-off-by: Joe Jin <joe.jin@oracle.com>
Reviewed-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
Acked-by: Wei Liu <wei.liu2@citrix.com>