virttest.video_maker: Fix the counter bug

When the test case destroy the old vm and recreate new, there will
be two different folders which has the screendump pictures,
however, when the video make try get the pictures in the new folder,
the index will be not 1

This patch is to find the smallest index in the folder.

Signed-off-by: Mike Qiu <qiudayu@linux.vnet.ibm.com>