qemu_extdevice: Add a comment into qemuExtDevicesSetupCgroup()

The way setting up CGroups for external helpers work, is:
qemuExtDevicesHasDevice() is called first to determine whether
there is a helper process running, the CGroup controller is
created and then qemuExtDevicesSetupCgroup() is called to place
helpers into the CGroup. But when one reads just
qemuExtDevicesSetupCgroup() it's easy to miss this hidden logic.
Therefore, add a warning at the beginning of the function.

Signed-off-by: Michal Privoznik <mprivozn@redhat.com>
Reviewed-by: Laine Stump <laine@redhat.com>

src/qemu/qemu_extdevice.c

@@ -331,6 +331,9 @@ qemuExtDevicesSetupCgroup(virQEMUDriver *driver,
     virDomainDef *def = vm->def;
     size_t i;
 
+    /* Don't forget to adjust qemuExtDevicesHasDevice() accordingly.
+     * Otherwise, this function might not be called at all. */
+
     if (qemuDBusSetupCgroup(driver, vm, cgroup) < 0)
         return -1;