The ScExportTest::testConditionalFormatExportXLSX() fails on Windows
because of how SfxMedium handles its streams:
1. SfxMedium::GetOutputStorage() creates some temp file
2. SfxMedium::GetMedium_Impl() opens a XStream on the temp file
3. SfxMedium::GetOutStream() wants to open a SvFileStream on the temp
   file, but because the file is already open and the sharing options
   are set to deny sharing, opening fails with ERROR_SHARING_VIOLATION

Prevent that by re-using the already open XStream in GetOutStream.
Hopefully this does not break anything, and there is already a comment
in CloseInStream_Impl() indicating that m_pOutStream and xStream are
related.

(interestingly ERROR_SHARING_VIOLATION is documented to occur if
_another_ process has the file open, but evidently it happens here on
NT 6.1 for the same process...)

Change-Id: I6d2ec36fd45a0317e947ddfb436472a8b86fbe26