virt: Rename virttest/nfs_unnittest.py to virttest/nfs_unittest.py

Because virttest/nfs_unnittest.py is end with _unnittest.py, not
_unittest.py, tool/run_unittest.py cannot find this module.

Signed-off-by: Li Yang <liyang.fnst@cn.fujitsu.com>
Merge pull request #813 from liyangfnst/nfs_unittest

virt: Rename virttest/nfs_unnittest.py to virttest/nfs_unittest.py