vfio-pci: Fix possible integer overflow

The VFIO_DEVICE_SET_IRQS ioctl takes a start and count parameter, both
of which are unsigned.  We attempt to bounds check these, but fail to
account for the case where start is a very large number, allowing
start + count to wrap back into the valid range.  Bounds check both
start and start + count.

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