vfio-pci: Replace 'void __user *' with proper types in the ioctl functions

This makes the code clearer and replaces a few places trying to access a
flex array with an actual flex array.

Reviewed-by: Kevin Tian <kevin.tian@intel.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>
Link: https://lore.kernel.org/r/4-v2-0f9e632d54fb+d6-vfio_ioctl_split_jgg@nvidia.com


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