virt.tests.nicdriver_unload : modify the file transfer method

Now this script use utils_test.run_file_transfer to do file transfer,
unfortunately this method is not suitable for this test, because the guest
nic is repeatedly down/up during test, some times the guest will try to ssh
the host when the nic is down, this will cause error like no route.

Signed-off-by: Yunping Zheng <yunzheng@redhat.com>