vfio: Introduce a new header file for VFIOIOMMUFD declarations

vfio: Introduce a new header file for VFIOIOMMUFD declarations

Gather all VFIOIOMMUFD related declarations introduced by commits
5ee3dc7af785 ("vfio/iommufd: Implement the iommufd backend") and
5b1e96e65403 ("vfio/iommufd: Introduce auto domain creation") into
"vfio-iommufd.h". This to reduce exposure of VFIO internals in
"hw/vfio/vfio-common.h".

Cc: Joao Martins <joao.m.martins@oracle.com>
Cc: Yi Liu <yi.l.liu@intel.com>
Reviewed-by: John Levon <john.levon@nutanix.com>
Link: https://lore.kernel.org/qemu-devel/20250318095415.670319-10-clg@redhat.com
Reviewed-by: Joao Martins <joao.m.martins@oracle.com>
Reviewed-by: Zhenzhong Duan <zhenzhong.duan@intel.com>
Link: https://lore.kernel.org/qemu-devel/20250326075122.1299361-11-clg@redhat.com
Signed-off-by: Cédric Le Goater <clg@redhat.com>