virt.tests.jumbo : Fix some issues of jumbo test

make a check before delete an arp item.
using 'get_win_wol' to get the drive number of win_utils

Signed-off-by: Yunping Zheng <yunzheng@redhat.com>
Merge pull request #664 from humanux/jumbo

virt.tests.jumbo : Fix some issues of jumbo test