vfio-pci: Further fix BAR size overflow

In an analysis by Laszlo, the resulting type of our calculation for
the end of the MSI-X table, and thus the start of memory after the
table, is uint32_t.  We're therefore not correctly preventing the
corner case overflow that we intended to fix here where a BAR >=4G
could place the MSI-X table to end exactly at the 4G boundary.  The
MSI-X table offset is defined by the hardware spec to 32bits, so we
simply use a cast rather than changing data structure types.  This
scenario is purely theoretically, typically the MSI-X table is located
at the front of the BAR.

Reported-by: Laszlo Ersek <lersek@redhat.com>
Reviewed-by: Laszlo Ersek <lersek@redhat.com>
Signed-off-by: Alex Williamson <alex.williamson@redhat.com>

hw/vfio/pci.c

@@ -2400,7 +2400,7 @@ static void vfio_map_bar(VFIOPCIDevice *vdev, int nr)
     if (vdev->msix && vdev->msix->table_bar == nr) {
         uint64_t start;
 
-        start = HOST_PAGE_ALIGN(vdev->msix->table_offset +
+        start = HOST_PAGE_ALIGN((uint64_t)vdev->msix->table_offset +
                                 (vdev->msix->entries * PCI_MSIX_ENTRY_SIZE));
 
         size = start < bar->region.size ? bar->region.size - start : 0;