virt.tests.nicdriver_unload : modify the file transfer method

Currently, this script uses utils_test.run_file_transfer transfer files
host->guest. Unfortunately, this method is not suitable for this test, since
the guest nic is repeatedly down/up during test, some times causing the host to
ssh the guest when the nic is down, causing errors like no route. Improve the
handling of connection failures, so we have more robust testing.

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