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

drivers/vfio/Makefile

 # SPDX-License-Identifier: GPL-2.0
 vfio_virqfd-y := virqfd.o
 
+vfio-y += vfio_main.o
+
 obj-$(CONFIG_VFIO) += vfio.o
 obj-$(CONFIG_VFIO_VIRQFD) += vfio_virqfd.o
 obj-$(CONFIG_VFIO_IOMMU_TYPE1) += vfio_iommu_type1.o