qmemman: improve check whether VM is still running

In some cases it may happen that qmemman or other application using
xenstore will re-create VM directory in xenstore just after VM was
destroyed. For example when multiple VMs was destroyed at the same time,
but qmemman will kick off just at first @releaseDomain event - other VMs
will still be there (at xenstore-list time). This means that qmemman
will consider them when redistributing memory (of just destroyed one),
so will update memory/target entry of every "running" VM. And at this
point it may recreate VM directory of another already destroyed VM.

Generally fixing this race condition would require running all the
operations (from xenstore-ls, to setting memory/target) in a single
xenstore transaction. But this can be lengthly process. And if any other
modification happens in the meantime, transaction will rejected and
qmemman would need to redo all the changes. Not worth the effort.

Fixes QubesOS/qubes-issues#1409

qmemman/qmemman_server.py

@@ -74,6 +74,16 @@ class XS_Watcher:
         self.log.debug('domain_list_changed(param={!r})'.format(param))
 
         curr = self.handle.ls('', '/local/domain')
+        # check if domain is really there, it may happen that some empty
+        # directories are left in xenstore
+        curr = filter(
+            lambda x:
+            self.handle.read('',
+                             '/local/domain/{}/domid'.format(x)
+                             ) is not None,
+            curr
+        )
+
         self.log.debug('curr={!r}'.format(curr))
 
         if curr == None: