runqemu: Ensure we cleanup snapshot files after image run

We need to cleanup snapshot files if we make a copy of them to ensure
the tmpfs doesn't run out of space. There is already NFS code needing
this so make it a generic code path.

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
(cherry picked from commit a3e0eec5)
Signed-off-by: Anuj Mittal <anuj.mittal@intel.com>