QLIST_REMOVE() assumes the element is in a list.  It also leaves the
element's linked list pointers dangling.

Introduce a safe version of QLIST_REMOVE() and convert open-coded
instances of this pattern.

Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Reviewed-by: Sergio Lopez <slp@redhat.com>
Message-id: 20200214171712.541358-4-stefanha@redhat.com
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>