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CA-292144 Avoid having an invalid leaf while taking a snapshot

After the original leaf has been renamed, td-util is asked to create a
snapshot with the filename it originally had. This means that after
td-util creates the file there is a brief window before it finishes
populating it when an attempt to query it will fail because the file
contents are invalid.

If we instead create the new leaf with a temporary filename
(foo.vhd.new) and rename it afterwards, this window is converted into
one during which the file does not exist, a condition in which a query
attempt will be retried for up to 20 seconds.

If necessary, the window can be shrunk further by using links so that
the old and new names of the original leaf exist simultaneously until it
is time to rename the new leaf into place (this is necessary because
td-util's snapshot function requires the parent to exist and be valid,
otherwise we could just create the snapshot first).

Signed-off-by: Tim Smith <tim.smith@citrix.com>

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