virt.tests: fix no ip address for extra nics

Some guest (eg, sles, ubt) don't configure IP address for extra nics and
ping.multi_nic always failed because no IP for extra nic; fix this issue
by send pre_cmd to guest for asking IP from dhcp server before start test;

Signed-off-by: Xu Tian <xutian@redhat.com>
Merge pull request #1188 from xutian/multi_nic_ping

virt.tests: fix no ip address for extra nics