virttest: Move nic_script under tap device branch in add_net

nic_script should be parameter for tap devices, so move it to
the right place. And now we can not control guest to use nic_script
or fds with cfg files as tapfds are automatic get by framework functions
only depends on the host env. So update the judgement as if we setup
nic_scritp, will let guest use script, otherwise will try to use fds.

Signed-off-by: Yiqiao Pu <ypu@redhat.com>
Merge pull request #1259 from ypu/nic_script

virttest: Move nic_script under tap device branch in add_net