Issue 154 (http://code.google.com/p/ganeti/issues/detail?id=154

)
reported an “Operation not supported” error when writing instance
exports to a mounted CIFS filesystem. Experimentation showed the error
to only occur when using rename(2) on an opened file. Various references
on the web confirmed this observation. Whether or not the problem occurs
can also depend on the CIFS server implementation. In issue 154 it was
Windows 2008 R2.

While not solving all cases, closing the file before renaming helps
alleviating the issue a bit. Unittests are updated.

Signed-off-by: Michael Hanselmann <hansmi@google.com>
Reviewed-by: Iustin Pop <iustin@google.com>