virttest.utils_net: fix remote runner error

If the runner is remote then runner(cmd) will return command status and result, and we only want to find match text in stdout, so it should be runner(cmd).stdout for remote runner.
@ypu, in fact, I only tried to use get_net_if_addrs() in virttest/utils_net.py, as I said, the result will return contents like this:

 * Command:
    ip addr show eth0 1>/tmp/tmpEm9MU3 2>/tmp/tmpn6lqz8
Exit status: 0
Duration: 0

stdout:
2: eth0: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc pfifo_fast state UP qlen 1000
    link/ether 10:65:4b:5c:b9:e5 brd ff:ff:ff:ff:ff:ff
    inet 10.11.5.33/22 brd 10.11.5.255 scope global eth0
    inet6 3ffg::102/64 scope global
       valid_lft forever preferred_lft forever
    inet6 be80::1230:4bff:fe5c:b9e5/64 scope link
       valid_lft forever preferred_lft forever
And then RE search will be failed, I think it should be the same issues for get_net_if(), if not, I will only change get_net_if_addrs(), thanks for your comments.