Always keep screendumps

screendumps are important, the current
policy of saving it when case error only is
not enough, if a VM network works fine and
case pass, but VM vga malfunction, deleted
them would potential miss bugs

Signed-off-by: Xiaoqing Wei <xwei@redhat.com>
Merge pull request #1497 from xiaoqing-wei/always_screendump

Always keep screendumps