Fix starting domains when kernel has no cgroups support

Found that I was unable to start existing domains after updating
to a kernel with no cgroups support

  # zgrep CGROUP /proc/config.gz
  # CONFIG_CGROUPS is not set
  # virsh start test
  error: Failed to start domain test
  error: Unable to initialize /machine cgroup: Cannot allocate memory

virCgroupPartitionNeedsEscaping() correctly returns errno (ENOENT) when
attempting to open /proc/cgroups on such a system, but it was being
dropped in virCgroupSetPartitionSuffix().

Change virCgroupSetPartitionSuffix() to propagate errors returned by
its callees.  Also check for ENOENT in qemuInitCgroup() when determining
if cgroups support is available.
(cherry picked from commit bbe97ae9)

src/qemu/qemu_cgroup.c

@@ -415,7 +415,8 @@ int qemuInitCgroup(virQEMUDriverPtr driver,
         if (rc != 0) {
             if (rc == -ENXIO ||
                 rc == -EPERM ||
-                rc == -EACCES) { /* No cgroups mounts == success */
+                rc == -EACCES ||
+                rc == -ENOENT) { /* No cgroups mounts == success */
                 VIR_DEBUG("No cgroups present/configured/accessible, ignoring error");
                 goto done;
             }

src/util/vircgroup.c

@@ -1180,14 +1180,14 @@ static int virCgroupPartitionEscape(char **path)
     return 0;
 }
 
-static char *virCgroupSetPartitionSuffix(const char *path)
+static int virCgroupSetPartitionSuffix(const char *path, char **res)
 {
     char **tokens = virStringSplit(path, "/", 0);
     size_t i;
-    char *ret = NULL;
+    int ret = -1;
 
     if (!tokens)
-        return NULL;
+        return ret;
 
     for (i = 0 ; tokens[i] != NULL ; i++) {
         /* Whitelist the 3 top level fixed dirs
@@ -1206,20 +1206,27 @@ static char *virCgroupSetPartitionSuffix(const char *path)
             !strchr(tokens[i], '.')) {
             if (VIR_REALLOC_N(tokens[i],
                               strlen(tokens[i]) + strlen(".partition") + 1) < 0) {
+                ret = -ENOMEM;
                 virReportOOMError();
                 goto cleanup;
             }
             strcat(tokens[i], ".partition");
         }
 
-        if (virCgroupPartitionEscape(&(tokens[i])) < 0) {
-            virReportOOMError();
+        ret = virCgroupPartitionEscape(&(tokens[i]));
+        if (ret < 0) {
+            if (ret == -ENOMEM)
+                virReportOOMError();
             goto cleanup;
         }
     }
 
-    if (!(ret = virStringJoin((const char **)tokens, "/")))
+    if (!(*res = virStringJoin((const char **)tokens, "/"))) {
+        ret = -ENOMEM;
         goto cleanup;
+    }
+
+    ret = 0;
 
 cleanup:
     virStringFreeList(tokens);
@@ -1254,9 +1261,9 @@ int virCgroupNewPartition(const char *path,
 
     /* XXX convert all cgroups APIs to use error report
      * APIs instead of returning errno */
-    if (!(newpath = virCgroupSetPartitionSuffix(path))) {
+    rc = virCgroupSetPartitionSuffix(path, &newpath);
+    if (rc < 0) {
         virResetLastError();
-        rc = -ENOMEM;
         goto cleanup;
     }