qemu.tests.multi_nics_verify: Mofidy the method of get guest nics count

The output of ifconfig is different between different os,
so some time the regex can not get the nics number in the guest.
This patch try to fix this issue

this patch depend on : https://github.com/autotest/virt-test/pull/686
Signed-off-by: Yunping Zheng <yunzheng@redhat.com>