tools/python: Correct the install path of the python scripts

tools/python: Correct the install path of the python scripts

c/s 74a7c7a was actually written long before c/s 2c96ecb "Use
configure --libexecdir=BASEDIR to set LIBEXEC", and I didn't notice
during development because of the method I was using to run a legacy
and migration v2 libxl toolstack side-by-side.

The result is that the conversion script gets installed in the root of
the output tree, rather than in the location libxl is expecting to
find it.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
CC: Ian Campbell <Ian.Campbell@citrix.com>
CC: Ian Jackson <Ian.Jackson@eu.citrix.com>
CC: Wei Liu <wei.liu2@citrix.com>
Acked-by: Ian Campbell <ian.campbell@citrix.com>