qemu: fix an off-by-one error in qemuDomainGetPercpuStats

The max value of number of cpus to compute(id) should not
be equal or greater than max cpu number.
The bug ocurrs when id value is equal to max cpu number which
leads to the off-by-one error in the following for loop.

 # virsh  cpu-stats guest --start 1
 error: Failed to virDomainGetCPUStats()

 error: internal error cpuacct parse error

src/qemu/qemu_driver.c

@@ -14331,9 +14331,9 @@ qemuDomainGetPercpuStats(virDomainObjPtr vm,
     param_idx = 0;
 
     /* number of cpus to compute */
-    id = max_id;
-
-    if (max_id - start_cpu > ncpus - 1)
+    if (start_cpu >= max_id - ncpus)
+        id = max_id - 1;
+    else
         id = start_cpu + ncpus - 1;
 
     for (i = 0; i <= id; i++) {