Load Qubes DB python module only when needed.

This is temporary band aid for the python module problems.
This should also slightly speed up some tools (not all uses Qubes DB).

core-modules/000QubesVm.py

@@ -39,7 +39,6 @@ from qubes import qmemman_algo
 import libvirt
 import warnings
 
-from qubes.qdb import QubesDB
 from qubes.qubes import dry_run,vmm
 from qubes.qubes import register_qubes_vm_class
 from qubes.qubes import QubesVmCollection,QubesException,QubesHost,QubesVmLabels
@@ -664,6 +663,7 @@ class QubesVm(object):
     def qdb(self):
         if self._qdb_connection is None:
             if self.is_running():
+                from qubes.qdb import QubesDB
                 self._qdb_connection = QubesDB(self.name)
         return self._qdb_connection