vfio/type1: silence integer overflow warning

I get a static checker warning about the potential integer overflow if
we add "unmap->iova + unmap->size".  The integer overflow isn't really
harmful, but we may as well fix it.  Also unmap->size gets truncated to
size_t when we pass it to vfio_find_dma() so we could check for too high
values of that as well.

Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Alex Williamson <alex.williamson@redhat.com>