qemu_cgroup: Allow/disallow devmapper control iff available

https://bugzilla.redhat.com/show_bug.cgi?id=1591732

On kernels without device mapper support there won't be
/dev/mapper/control. Therefore it doesn't make much sense to
put it into devices CGroup.
Signed-off-by: NMichal Privoznik <mprivozn@redhat.com>
Reviewed-by: NJán Tomko <jtomko@redhat.com>

src/qemu/qemu_cgroup.c

@@ -129,6 +129,7 @@ qemuSetupImageCgroupInternal(virDomainObjPtr vm,
     }
 
     if (virStoragePRDefIsManaged(src->pr) &&
+        virFileExists(DEVICE_MAPPER_CONTROL_PATH) &&
         qemuSetupImagePathCgroup(vm, DEVICE_MAPPER_CONTROL_PATH, false) < 0)
         return -1;
 
@@ -163,6 +164,7 @@ qemuTeardownImageCgroup(virDomainObjPtr vm,
         return 0;
     }
 
+    if (virFileExists(DEVICE_MAPPER_CONTROL_PATH)) {
         for (i = 0; i < vm->def->ndisks; i++) {
             virStorageSourcePtr diskSrc = vm->def->disks[i]->src;
 
@@ -183,7 +185,7 @@ qemuTeardownImageCgroup(virDomainObjPtr vm,
             if (ret < 0)
                 return ret;
         }
-
+    }
 
     VIR_DEBUG("Deny path %s", src->path);