qemu : make virtual_nic_sendbuffer test support windows guest

This patch make virtual_nic_sendbuffer test support windows guest.
Windows guest:
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 programe can run normally in your guest.

Linux guest:
    make the udt_url is valid git repo