block/vdi: Limit maximum size even futher

The block layer read and write functions do not like requests which are
bigger than INT_MAX bytes. Since the VDI bmap is read and written in a
single operation, its size is therefore limited accordingly. This
reduces the maximum VDI image size supported by QEMU to half of what it
currently is (down to approximately 512 TB).

The VDI test 084 has to be adapted accordingly. Actually, one could
clearly see that it was broken from the "Could not open
'TEST_DIR/t.IMGFMT': Invalid argument" line for an image which was
supposed to work just fine.

Signed-off-by: Max Reitz <mreitz@redhat.com>