vfio/platform: Use vfio_[attach/detach]_device

Let the vfio-platform device use vfio_attach_device() and
vfio_detach_device(), hence hiding the details of the used
IOMMU backend.

Drop the trace event for vfio-platform as we have similar
one in vfio_attach_device.

Signed-off-by: Eric Auger <eric.auger@redhat.com>
Signed-off-by: Yi Liu <yi.l.liu@intel.com>
Signed-off-by: Zhenzhong Duan <zhenzhong.duan@intel.com>
Reviewed-by: Cédric Le Goater <clg@redhat.com>
Signed-off-by: Cédric Le Goater <clg@redhat.com>