qemu: fix improper initialization of cgroupControllers bitmap

virQEMUDriverConfigNew() always initializes the bitmap in its
cgroupControllers member to -1 (i.e. all 1's).

Prior to commit a9331394, if qemu.conf had a line with
"cgroup_controllers", cgroupControllers would get reset to 0 before
going through a loop setting a bit for each named cgroup controller.
commit a9331394 left out the "reset to 0" part, so cgroupControllers
would always be -1; if you didn't want a controller included, there
was no longer a way to make that happen.

This was discovered by users who were using qemu commandline
passthrough to use the "input-linux" method of directing
keyboard/mouse input to a virtual machine:

 https://www.redhat.com/archives/vfio-users/2016-April/msg00105.html

Here's the first report I found of the problem encountered after
upgrading libvirt beyond v2.0.0:

 https://www.redhat.com/archives/vfio-users/2016-August/msg00053.html

Thanks to sL1pKn07 SpinFlo <sl1pkn07@gmail.com> for bringing the
problem up in IRC, and then taking the time to do a git bisect and
find the patch that started the problem.

src/qemu/qemu_conf.c

@@ -613,15 +613,18 @@ int virQEMUDriverConfigLoadFile(virQEMUDriverConfigPtr cfg,
                                   &controllers) < 0)
         goto cleanup;
 
-    for (i = 0; controllers != NULL && controllers[i] != NULL; i++) {
-        int ctl;
-        if ((ctl = virCgroupControllerTypeFromString(controllers[i])) < 0) {
-            virReportError(VIR_ERR_CONF_SYNTAX,
-                           _("Unknown cgroup controller '%s'"),
-                           controllers[i]);
-            goto cleanup;
+    if (controllers) {
+        cfg-> cgroupControllers = 0;
+        for (i = 0; controllers[i] != NULL; i++) {
+            int ctl;
+            if ((ctl = virCgroupControllerTypeFromString(controllers[i])) < 0) {
+                virReportError(VIR_ERR_CONF_SYNTAX,
+                               _("Unknown cgroup controller '%s'"),
+                               controllers[i]);
+                goto cleanup;
+            }
+            cfg->cgroupControllers |= (1 << ctl);
         }
-        cfg->cgroupControllers |= (1 << ctl);
     }
 
     if (virConfGetValueStringList(conf,  "cgroup_device_acl", false,