qom: Split object_property_set_link()

The path resolution logic in object_property_set_link() should be a
separate function.  This makes the code easier to read and maintain.

Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Reviewed-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Andreas Färber <afaerber@suse.de>
qom: Don't make link NULL on object_property_set_link() failure

The error behavior of object_property_set_link() is dangerous.  It sets
the link property object to NULL if an error occurs.  A setter function
should either succeed or fail, it shouldn't leave the value NULL on
failure.

Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Reviewed-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Andreas Färber <afaerber@suse.de>
qom: Make QOM link property unref optional

Some object_property_add_link() callers expect property deletion to
unref the link property object.  Other callers expect to manage the
refcount themselves.  The former are currently broken and therefore leak
the link property object.

This patch adds a flags argument to object_property_add_link() so the
caller can specify which refcount behavior they require.  The new
OBJ_PROP_LINK_UNREF_ON_RELEASE flag causes the link pointer to be
unreferenced when the property is deleted.

This fixes refcount leaks in qdev.c, xilinx_axidma.c, xilinx_axienet.c,
s390-virtio-bus.c, virtio-pci.c, virtio-rng.c, and ui/console.c.

Rationale for refcount behavior:

 * hw/core/qdev.c
   - bus children are explicitly unreferenced, don't interfere
   - parent_bus is essentially a read-only property that doesn't hold a
     refcount, don't unref
   - hotplug_handler is leaked, do unref

 * hw/dma/xilinx_axidma.c
   - rx stream "dma" links are set using set_link, therefore they
     need unref
   - tx streams are set using set_link, therefore they need unref

 * hw/net/xilinx_axienet.c
   - same reasoning as hw/dma/xilinx_axidma.c

 * hw/pcmcia/pxa2xx.c
   - pxa2xx bypasses set_link and therefore does not use refcounts

 * hw/s390x/s390-virtio-bus.c
 * hw/virtio/virtio-pci.c
 * hw/virtio/virtio-rng.c
 * ui/console.c
   - set_link is used and there is no explicit unref, do unref

Cc: Peter Crosthwaite <peter.crosthwaite@petalogix.com>
Cc: Alexander Graf <agraf@suse.de>
Cc: Anthony Liguori <aliguori@amazon.com>
Cc: "Michael S. Tsirkin" <mst@redhat.com>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Reviewed-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Andreas Färber <afaerber@suse.de>
qom: Add check() argument to object_property_add_link()

There are currently three types of object_property_add_link() callers:

1. The link property may be set at any time.
2. The link property of a DeviceState instance may only be set before
   realize.
3. The link property may never be set, it is read-only.

Something similar can already be achieved with
object_property_add_str()'s set() argument.  Follow its example and add
a check() argument to object_property_add_link().

Also provide default check() functions for case #1 and #2.  Case #3 is
covered by passing a NULL function pointer.

Cc: Peter Crosthwaite <peter.crosthwaite@petalogix.com>
Cc: Alexander Graf <agraf@suse.de>
Cc: Anthony Liguori <aliguori@amazon.com>
Cc: "Michael S. Tsirkin" <mst@redhat.com>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Reviewed-by: Paolo Bonzini <pbonzini@redhat.com>
[AF: Tweaked documentation comment]
Signed-off-by: Andreas Färber <afaerber@suse.de>
virtio-rng: Avoid default_backend refcount leak

QOM child properties take a reference to the object and release it when
the property is deleted.  Therefore we should unref the default_backend
after we have added it as a child property.

Cc: KONRAD Frederic <fred.konrad@greensocs.com>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Reviewed-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Andreas Färber <afaerber@suse.de>