qemu: don't fail update netdev on bridge detach failure

When a network device's bridge connection is changed by
virDomainUpdateDevice, libvirt first removes the netdev's tap from its
old bridge, then adds it to the new bridge. Sometimes, due to a
network being destroyed while a guest device is still attached, the
tap may already be "removed" from the old bridge (or the old bridge
may not even exist any more); the existing code was needlessly failing
the update when this happened, making it impossible to recover from
the situation without completely detaching (i.e. removing) the netdev
from the guest and re-attaching.

Instead of failing the entire operation when removal of the tap from
the old bridge fails, this patch changes qemuDomainChangeNetBridge to
just log a warning and continue, allowing a reasonable recover from
the situation.

(you'll appreciate this change if you ever accidentally destroy a
network while your guests are still using it).

src/qemu/qemu_hotplug.c

@@ -1306,8 +1306,14 @@ qemuDomainChangeNetBridge(virConnectPtr conn,
     if (oldbridge) {
         ret = virNetDevBridgeRemovePort(oldbridge, olddev->ifname);
         virDomainAuditNet(vm, olddev, NULL, "detach", ret == 0);
-        if (ret < 0)
-            goto cleanup;
+        if (ret < 0) {
+            /* warn but continue - possibly the old network
+             * had been destroyed and reconstructed, leaving the
+             * tap device orphaned.
+             */
+            VIR_WARN("Unable to detach device %s from bridge %s",
+                     olddev->ifname, oldbridge);
+        }
     }
 
     ret = virNetDevBridgeAddPort(newbridge, olddev->ifname);