vbox: don't register NULL driver

We were missing check for the fact that the storage driver was found and
in case there is no vbox storage driver available, daemon raised the
following error each start:

  error : virRegisterStorageDriver:592 : driver in
  virRegisterStorageDriver must not be NULL

Fixing this makes the condition unified with networkDriver registration
in vbox as well.
Signed-off-by: NMartin Kletzander <mkletzan@redhat.com>

src/vbox/vbox_driver.c

@@ -74,7 +74,7 @@ int vboxStorageRegister(void)
     if (VBoxCGlueInit(&uVersion) == 0)
         storageDriver = vboxGetStorageDriver(uVersion);
 
-    if (virRegisterStorageDriver(storageDriver) < 0)
+    if (storageDriver && virRegisterStorageDriver(storageDriver) < 0)
         return -1;
     return 0;
 }