migration: No need to return the size of the cache

migration: No need to return the size of the cache

After the previous commits, we make sure that the value passed is
right, or we just drop an error.  So now we return if there is one
error or we have setup correctly the value passed.

Signed-off-by: Juan Quintela <[email protected]>
Reviewed-by: Dr. David Alan Gilbert <[email protected]>
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Improve error messasge
Return 0 always for success