[libvirt] Selective block device migration implementation

[Pavel Boldin]

> That's why I've chosen to work purely with disk target at NBD level. We
> have strong rules what characters can occur there. Moreover, it's fairly
> easy to derive qemu disk ID from the target. Oh, and we require targets
> to be unique throughout the domain. So I think it's the best option for
> the extension you're planning.
>
Bad news is NBD is not enabled for tunnelled migration:
(src/qemu/qemu_migration.c, line 2635).
        /* TODO support NBD for TUNNELLED migration */
        if (flags & VIR_MIGRATE_TUNNELLED) {
            VIR_WARN("NBD in tunnelled migration is currently not
supported");
        } else {
            cookieFlags |= QEMU_MIGRATION_COOKIE_NBD;
            priv->nbdPort = 0;
        }

So, we will need to implement this first for my code path, since all the
migrations started by the Nova a tunnelled.



> Hm.. what about this, introduce just this new function:
>
> virTypedParamsGetArrayForKey(.., const char *key, ...)
>

Since we are using libvirt-python in nova I will have to change the way
python bindings convert input `dict' to a TypedParams. For this, the
`virPyDictToTypedParams' would have to be changed.

One cannot pass a multiple values with the same key in Python so
`virPyDictToTypedParams' will have to translate dict(migrate_disk=[1,2,3])
into TypedParams {("migrate_disk", 1), ("migrate_disk", 2),
("migrate_disk", 3)}.

Pavel
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