In fcd93edd00a8c9b08da3994d8bee9fa3809c3d93
"netboot: Rework interface to host_netboot_file (was host_pxefile)"
we unintentionally dropped $ho->{Tftp}{Path} from the
effective path. Reinstate it.
Signed-off-by: Ian Jackson <ian.jackson@eu.citrix.com>