vbox: add test for shared folders

Issue #1176
vbox: workaround for bug in Windows guest addition

In file

src/VBox/Additions/WINNT/SharedFolders/driver/file.c

the function

static int vbsfTransferCommon(VBSFTRANSFERCTX *pCtx)

in the

VbglR0CanUsePhysPageList()

branch does not correctly evaluate the read or written bytes from
the VMM. It ever assumes that whole pages are read/written.

Workaround the bug in the Windows guest additions of Vbox until fixed
upstream by filling up the read/write buffer completely within the VMM code
of Vbox.

Fixes #1176