configure.ac: Require Python when building client
configure.ac: Require Python when building client

- The "none" VMM driver requires Python and is installed with
--enable-client.  Have configure.ac test for Python when --enable-client
is specified.

cmusatyalab/openisr,cmusatyalab/openisr,cmusatyalab/openisr,cmusatyalab/openisr,cmusatyalab/openisr