xend: Prevent XenD touching externally managed bridges

xend: Prevent XenD touching externally managed bridges

With current XenD 3.0.4 or later try the following:

    brctl addbr demo
    ifconfig demo up

    /etc/init.d/xend start
    /etc/init.d/xend stop

    ifconfig demo down
    brctl delbr demo

Now, start XenD again....

    /etc/init.d/xend start

And watch in horror as it re-creates your 'demo' bridge.

The problem is that the 'XendNetwork' class does not distinguish
between bridge devices that it is managing (ie those created via
XenAPI) and those which it does not manage (ie those created by OS
distro init scripts, or by apps like  libvirt).

While initially I thought I could just make XenD ignore
externally-managed bridges completely, it seems to needs to know about
them otherwise it can't hook up guest VIFs to them correctly. So the
attached patch adds a 'managed' flag to the XendNetwork
class. Externally managed bridges have this set to False. At startup
XenD will now only re-create bridge devices which have the 'managed'
flag set to 'True'  - ie those created via XenAPI.

Signed-off-by: Daniel P. Berrange <berrange@redhat.com>