Change 7-Zip to new URL

scripts/vm-guest-tools.bat

 if not exist "C:\Windows\Temp\7z920-x64.msi" (
-    powershell -Command "(New-Object System.Net.WebClient).DownloadFile('http://downloads.sourceforge.net/sevenzip/7z920-x64.msi', 'C:\Windows\Temp\7z920-x64.msi')" <NUL
+    powershell -Command "(New-Object System.Net.WebClient).DownloadFile('http://www.7-zip.org/a/7z920-x64.msi', 'C:\Windows\Temp\7z920-x64.msi')" <NUL
 )
 msiexec /qb /i C:\Windows\Temp\7z920-x64.msi