Import VFIO headers

To ensure consistency between kvmtool and the kernel, import the UAPI
headers of the VFIO version we implement. This is from Linux v4.12.
Reviewed-by: Punit Agrawal <punit.agrawal@arm.com>
Signed-off-by: Jean-Philippe Brucker <jean-philippe.brucker@arm.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>