Fix bug in drbd8 replace disks on current nodes

Currently the drbd8 replace-disks on the same node (i.e. -p or -s) has
a bug in that it does modify the instance disk temporarily before
changing it back to the same value. However, we don't need to, and
shouldn't do that: what this operation do is simply change the LVM
configuration on the node, but otherwise the instance disks keep the
same configuration as before.

In the current code, this change back-and-forth is fine *unless* we
fail during attaching the new LVs to DRBD; in which case, we're left
with a half-modified disk, which is entirely wrong.

So we change the code in two ways:

- use temporary copies of the disk children in the old_lvs var
- stop updating disk.children

Which means that the instance should not be modified anymore (except
maybe for SetDiskID, which is a legacy and unfortunate decision that
will have to cleaned up sometime).

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Michael Hanselmann <hansmi@google.com>