improve injection diagnostics when nbd unavailable. Bug 755854

It's better to fail immediately with this improved diagnostic.
Relying on qemu-nbd to detect the issue (no device), is inefficient.

Change-Id: Ib124f6b83d8f04f0b4791c98912603f3499a9d2a

nova/virt/disk/nbd.py

@@ -54,6 +54,9 @@ class Mount(mount.Mount):
     _DEVICES = ['/dev/nbd%s' % i for i in range(FLAGS.max_nbd_devices)]
 
     def _allocate_nbd(self):
+        if not os.path.exists("/sys/block/nbd0"):
+            self.error = _('nbd unavailable: module not loaded')
+            return None
         while True:
             if not self._DEVICES:
                 # really want to log this info, not raise