configure: eliminate Python dependency for --help

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 <[email protected]>
Signed-off-by: Stefan Hajnoczi <[email protected]>
Reviewed-by: Peter Maydell <[email protected]>
Message-id: 20170328134418[email protected]
Signed-off-by: Stefan Hajnoczi <[email protected]>