virttest: Remove sit0 in get_linux_ifname of utils_net

sit0 is IPv6-in-IPv4 device added in system which not represent devices
added by vm.

Signed-off-by: Yiqiao Pu <ypu@redhat.com>
Merge pull request #1705 from ypu/sit0

virttest: Remove sit0 in get_linux_ifname of utils_net