VirDomainWrapper: avoid keeping libvirt object ref in locals

Complementary to the previous commit, avoid keeping keeping libvirt
object (function) reference in locals, which become closure for the
wrapper. Do getattr (again) when the actual call is made.
The primary reason for this change is to avoid leaking objects during
tests, but also this fixes a rare failure more - if external caller
would cache returned function, it wouldn't work after reconnection.
For example:

    # libvirt_domain is VirDomainWrapper() object
    >>> func = libvirt_domain.pause
    >>> func()
    >>> func()

If reconnection happens during the first call (or between calls), the
second call would use stale function reference.

qubes/app.py

@@ -104,12 +104,13 @@ def __getattr__(self, attrname):
         @functools.wraps(attr)
         def wrapper(*args, **kwargs):
             try:
-                return attr(*args, **kwargs)
+                return getattr(self._vm, attrname)(*args, **kwargs)
             except libvirt.libvirtError:
                 if self._reconnect_if_dead():
                     return getattr(self._vm, attrname)(*args, **kwargs)
                 raise
 
+        del attr
         return wrapper