configure: eliminate Python dependency for --help

The ./configure script should produce --help output even if Python is
not installed.

Listing trace backends is simple: show the names of all Python modules
in scripts/tracetool/backend/ whose source code contains 'PUBLIC =
True'.

Perform the backend enumeration in shell instead of Python so that we
can move the Python check until after ./configure --help.

Reported-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Message-id: 20170328134418.3426-1-stefanha@redhat.com
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>