build: fix tools/configure in case only python3 exists

build: fix tools/configure in case only python3 exists

Calling ./configure with python3 being there but no python,
tools/configure will fail. Fix that by defaulting to python and
falling back to python3 or python2.

While at it fix the use of non portable "type -p" by replacing it by
AC_PATH_PROG().

Signed-off-by: Juergen Gross <jgross@suse.com>
Acked-by: Wei Liu <wl@xen.org>
[ wei: run autogen.sh ]
Reviewed-by: Ian Jackson <ian.jackson@eu.citrix.com>