sample/mdev/mbochs: add mbochs_kunmap_dmabuf

There is no default implementation for dma_buf_ops->unmap.
So add a function unmapping the page, otherwise we'll leak them.

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Signed-off-by: Alex Williamson <alex.williamson@redhat.com>