vfio/ccw: Use functions for alloc/free of the vfio_ccw_private

Makes the code easier to understand what is memory lifecycle and what is
other stuff.

Reviewed-by: Eric Farman <farman@linux.ibm.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>
Link: https://lore.kernel.org/r/2-v4-cea4f5bd2c00+b52-ccw_mdev_jgg@nvidia.com


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