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>
