qcow2: Set the default cache-clean-interval to 10 minutes

qcow2: Set the default cache-clean-interval to 10 minutes

The default cache-clean-interval is set to 10 minutes, in order to lower
the overhead of the qcow2 caches (before the default was 0, i.e.
disabled).

* For non-Linux platforms the default is kept at 0, because
  cache-clean-interval is not supported there yet.

Signed-off-by: Leonid Bloch <[email protected]>
Reviewed-by: Alberto Garcia <[email protected]>
Reviewed-by: Kevin Wolf <[email protected]>
Signed-off-by: Kevin Wolf <[email protected]>