tests.mac_change : change mac when the interface is active

this patch make the mac_change test support changing mac when the
interface is active
this patch also modify the method of get drive number of win_utils

Signed-off-by: Yunping Zheng <yunzheng@redhat.com>
Merge pull request #658 from humanux/mac_change_v1

tests.mac_change : change mac when the interface is active