qcow2: Add overlap-check.template option

qcow2: Add overlap-check.template option

Being able to set the overlap-check option to a string and then refine
it via the overlap-check.* options is a nice idea for the command line
but does not work so well for non-flattened dicts. In that case, one can
only specify either but not both, so add a field to overlap-check.*
which does the same as directly specifying overlap-check but can be used
in conjunction with the other fields in non-flattened dicts.

Signed-off-by: Max Reitz <[email protected]>
Reviewed-by: Eric Blake <[email protected]>
Message-id: 1408557576[email protected]
Signed-off-by: Stefan Hajnoczi <[email protected]>