qemu: make virtual_nic_sendbuffer test support windows guests

This patch make virtual_nic_sendbuffer test support windows guest.

For Windows guests:

1. Download udt.sdk.4.8.win32.zip.
   link: http://sourceforge.net/projects/udt/files/udt/4.8/udt.sdk.4.8.win32.zip
2. Unzip it, and put the file in your guest disk or cdrom.
3. Make sure the program can run normally in your guest.

Linux guest:
    make the udt_url is a valid git repo

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