nwfilter: Partly initialize driver even for non-privileged users

https://bugzilla.redhat.com/show_bug.cgi?id=1211436

This reverts commit b7829f95.

The previous fix was not correct. Like everywhere else, a driver is a
global variable allocated in stateInitialize function (or something
similar for stateless drivers). Later, when a driver API is called,
it's possible that the global variable is accessed and dereferenced.
Now, some drivers require root privileges because they undertake some
actions reserved only for the system admin (e.g. manipulating host
firewall). And here's the trouble, the NWFilter state initializer
exited too early when finding out it's running unprivileged, leaving
the global NWFilter driver variable uninitialized. Any subsequent
API call that tried to lock the driver resulted in dereferencing the
driver and thus crash.

On the other hand, in order to not resurrect the bug the original
commit was fixing, Let's forbid the nwfilter define in session mode.
Signed-off-by: NMichal Privoznik <mprivozn@redhat.com>

Conflicts:
	src/nwfilter/nwfilter_driver.c: Context. Code changed a bit
        since 2013.

(cherry picked from commit 77d92e2e)

src/nwfilter/nwfilter_driver.c

@@ -176,9 +176,6 @@ nwfilterStateInitialize(bool privileged,
     char *base = NULL;
     DBusConnection *sysbus = NULL;
 
-    if (!privileged)
-        return 0;
-
     if (virDBusHasSystemBus() &&
         !(sysbus = virDBusGetSystemBus()))
         return -1;
@@ -193,6 +190,9 @@ nwfilterStateInitialize(bool privileged,
     driver->watchingFirewallD = (sysbus != NULL);
     driver->privileged = privileged;
 
+    if (!privileged)
+        return 0;
+
     nwfilterDriverLock();
 
     if (virNWFilterIPAddrMapInit() < 0)
@@ -535,6 +535,12 @@ nwfilterDefineXML(virConnectPtr conn,
     virNWFilterObjPtr nwfilter = NULL;
     virNWFilterPtr ret = NULL;
 
+    if (!driver->privileged) {
+        virReportError(VIR_ERR_OPERATION_INVALID, "%s",
+                       _("Can't define NWFilters in session mode"));
+        return NULL;
+    }
+
     nwfilterDriverLock();
     virNWFilterWriteLockFilterUpdates();
     virNWFilterCallbackDriversLock();