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[libvirt] RFC: a <uuid> for every <interface> in each domain

A couple of situations have come up recently that could be solved by
every interface in every domain always having a unique identifier
associated with it:

1) In order to properly track information as a guest is disconnected and
reconnected to an Open vSwitch bridge, the initial Open vSwitch support
added an "interfaceid" parameter to virtualport. If the interfaceid
isn't specified by the user when the interface is defined, one is
automatically generated and placed into the persistent definition.

But if the guest interface is just <interface type='network'>, it isn't
known at the time of definition whether this interface will be using an
Open vSwitch connection, or something else. Then, by the time we get
into the network driver and decide that we're going to use Open vSwitch,
it's too late to conveniently/cleanly generate an interfaceid and plug
it back into the <virtualport> of the interface's persistent config. The
result is that each time the guest is restarted, Open vSwitch gets a new
interfaceid for it, and can't properly track things.

If every interface had a uuid, the code that connects to an Open vSwitch
network could just use the interface's uuid if interfaceid wasn't specified.

2) I'm about to write some patches that will allow enacting config
changes to a network without needing to destroy/re-start the network.
One thing that came up while I was mulling over what could and couldn't
be modified without a restart is that it will be possible to allow
removing physical interfaces from a network's interface pool (used for
macvtap and hostdev modes), but only if that physical interface isn't
currently in use. There is a use counter on each interface so we can
easily tell when that situation occurs, but once we know of the failure,
we have no way of pointing to which guest is causing the problem. If
each interface had a uuid, we could save a list of the uuid's of all
interfaces currently connected to a particular physical interface, and
report that in the failure message. It would then be a fairly mechanical
task to find that uuid in the guests' config.

Does this sound like a reasonable idea? Any reasons *not* to do it?
Problems we'll need to take care of if we add it (for example, existing
guest interfaces will all need to get a uuid during the upgrade process,
similar to the way we add a mac address to all existing networks that
don't have one). Any other things you can think of doing with uuid if we
add one?

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