qemu: Log error if domain uses security driver which is not loaded

When starting a domain, if a domain specifies security drivers we do not have
loaded, we fail. However we don't check for this during
reconnect, so any operation relying on security driver functionality would fail.
If someone e.g. starts a domain with selinux driver loaded, then they change
the security driver to 'none' in config, restart the daemon and call dump/save/..,
QEMU will return an error.
As we shouldn't kill the domain, we should at least log an error to let the
user know that domain reconnect wasn't completely clean.

https://bugzilla.redhat.com/show_bug.cgi?id=1183893

src/qemu/qemu_process.c

@@ -3749,6 +3749,12 @@ qemuProcessReconnect(void *opaque)
         if ((qemuDomainAssignAddresses(obj->def, priv->qemuCaps, obj)) < 0)
             goto error;
 
+    /* if domain requests security driver we haven't loaded, report error, but
+     * do not kill the domain
+     */
+    ignore_value(virSecurityManagerCheckAllLabel(driver->securityManager,
+                                                 obj->def));
+
     if (virSecurityManagerReserveLabel(driver->securityManager, obj->def, obj->pid) < 0)
         goto error;