cgroup: read more data from cgroup cpuacct.usage_percpu

On NUMA machine, the length of string got from file
cpuacct.usage_percpu is quite large, so expand the
limit of 1024 bytes.

errors like:
Failed to read file \
'/cgroup/cpuacct/libvirt/qemu/rhel6q/cpuacct.usage_percpu': \
Value too large for defined data type

src/util/cgroup.c

@@ -360,7 +360,7 @@ static int virCgroupGetValueStr(virCgroupPtr group,
 
     VIR_DEBUG("Get value %s", keypath);
 
-    rc = virFileReadAll(keypath, 1024, value);
+    rc = virFileReadAll(keypath, 1024*1024, value);
     if (rc < 0) {
         rc = -errno;
         VIR_DEBUG("Failed to read %s: %m\n", keypath);