ch: set driver to NULL after freeing it

If the chStateInitialize method fails, we call chStateCleanup
which free's all global state. It fails to set the global
'ch_driver' to NULL, however, so a later attempt to open the
cloud hypervisor driver will succeed and then crash attempting
to access freed memory.

Reviewed-by: Michal Privoznik <mprivozn@redhat.com>
Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>

src/ch/ch_driver.c

@@ -827,6 +827,7 @@ static int chStateCleanup(void)
     virObjectUnref(ch_driver->config);
     virMutexDestroy(&ch_driver->lock);
     g_free(ch_driver);
+    ch_driver = NULL;
 
     return 0;
 }