util: Allow removing hash entries in virHashForEach

This fixes a possible crash of libvirtd during its startup. When qemu
driver reconnects to running domains, it iterates over all domain
objects in a hash. When reconnecting to an associated qemu monitor
fails and the domain is transient, it's immediately removed from the
hash. Despite the fact that it's explicitly forbidden to do so. If
libvirtd is lucky enough, virHashForEach will access random memory when
the callback finishes and the deamon will crash.

Since it's trivial to fix virHashForEach to allow removal of hash
entries while iterating through them, I went this way instead of fixing
qemuReconnectDomain callback (and possibly others) to avoid deleting the
entries.

src/util/hash.c

@@ -548,9 +548,8 @@ virHashRemoveEntry(virHashTablePtr table, const void *name)
  * @data: opaque data to pass to the iterator
  *
  * Iterates over every element in the hash table, invoking the
- * 'iter' callback. The callback must not call any other virHash*
- * functions, and in particular must not attempt to remove the
- * element.
+ * 'iter' callback. The callback is allowed to remove the element using
+ * virHashRemoveEntry but calling other virHash* functions is prohibited.
  *
  * Returns number of items iterated over upon completion, -1 on failure
  */
@@ -563,11 +562,12 @@ int virHashForEach(virHashTablePtr table, virHashIterator iter, void *data) {
     for (i = 0 ; i < table->size ; i++) {
         virHashEntryPtr entry = table->table + i;
         while (entry) {
+            virHashEntryPtr next = entry->next;
             if (entry->valid) {
                 iter(entry->payload, entry->name, data);
                 count++;
             }
-            entry = entry->next;
+            entry = next;
         }
     }
     return (count);