Comment 1 for bug 1682693

In http://git.openstack.org/cgit/openstack/nova/tree/nova/db/sqlalchemy/api.py#n2150
we did copied the filters dict
in http://git.openstack.org/cgit/openstack/nova/tree/nova/db/sqlalchemy/api.py#n2188 and
http://git.openstack.org/cgit/openstack/nova/tree/nova/db/sqlalchemy/api.py#n2209
we pop the first tag in tags to do SQL query, as we didn't do deep copy, the
"tags" or "not-tags" key will be still in filters dict but the value has been removed,
as we loop through every cell, Error will happen, and also, the result is incorrect.