virttest: issue fix for wait_for_get_address.

wait_for_get_address will raise error, when try to get guest
ipv4 address via serial session, this patch fix this issue

Signed-off-by: Yunping Zheng <yunzheng@redhat.com>
Merge pull request #1473 from humanux/ipaddr_issue_fix

virttest: issue fix for wait_for_get_address.