vfio: Move vfio.c to vfio_main.c

If a source file has the same name as a module then kbuild only supports
a single source file in the module.

Rename vfio.c to vfio_main.c so that we can have more that one .c file
in vfio.ko.

Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>
Signed-off-by: Yishai Hadas <yishaih@nvidia.com>
Link: https://lore.kernel.org/r/20220731125503.142683-5-yishaih@nvidia.com


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