guestfs: As virt-manager is multithreaded, don't use an atexit handler.

libguestfs adds an atexit handler to clean up handles on exit.  This
is convenient in single-threaded programs, but causes problems in
multithreaded programs, for example:

https://bugzilla.redhat.com/show_bug.cgi?id=1043520

Disable the atexit handler for all guestfs handles created by
virt-manager.

Note this change requires libguestfs >= 1.20.  However the change to
config.py should ensure that if an earlier version of libguestfs is
installed it should silently fall back to not using inspection.

virtManager/config.py

@@ -182,7 +182,7 @@ class vmmConfig(object):
         try:
             # Check we can open the Python guestfs module.
             from guestfs import GuestFS  # pylint: disable=F0401
-            GuestFS()
+            GuestFS(close_on_exit = False)
             return True
         except:
             return False

virtManager/inspection.py

@@ -163,7 +163,7 @@ class vmmInspection(vmmGObject):
                           vm.get_name())
             return None
 
-        g = GuestFS()
+        g = GuestFS(close_on_exit = False)
         prettyvm = conn.get_uri() + ":" + vm.get_name()
         ignore = vmuuid