iommufd: Move iommufd_sw_msi and related functions to driver.c

iommufd: Move iommufd_sw_msi and related functions to driver.c

To provide the iommufd_sw_msi() to the iommu core that is under a different
Kconfig, move it and its related functions to driver.c. Then, stub it into
the iommu-priv header. The iommufd_sw_msi_install() continues to be used by
iommufd internal, so put it in the private header.

Note that iommufd_sw_msi() will be called in the iommu core, replacing the
sw_msi function pointer. Given that IOMMU_API is "bool" in Kconfig, change
IOMMUFD_DRIVER_CORE to "bool" as well.

Since this affects the module size, here is before-n-after size comparison:
[Before]
   text	   data	    bss	    dec	    hex	filename
  18797	    848	     56	  19701	   4cf5	drivers/iommu/iommufd/device.o
    722	     44	      0	    766	    2fe	drivers/iommu/iommufd/driver.o
[After]
   text	   data	    bss	    dec	    hex	filename
  17735     808      56   18599    48a7 drivers/iommu/iommufd/device.o
   3020     180       0    3200     c80 drivers/iommu/iommufd/driver.o

Link: https://patch.msgid.link/r/374c159592dba7852bee20968f3f66fa0ee8ca93.1742871535.git.nicolinc@nvidia.com
Signed-off-by: Nicolin Chen <nicolinc@nvidia.com>
Reviewed-by: Kevin Tian <kevin.tian@intel.com>
Reviewed-by: Jason Gunthorpe <jgg@nvidia.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>