libxml2: fix python packaging for nativesdk

We enable the python module in nativesdk-libxml2, but the python binary
used is in the native sysroot and thus you get the module installed in
the wrong path. Even with that fixed the python files are still
unpackaged, so create an ${PN}-python package and add them to it. (This
does not affect the libxml target build at all since python is disabled
for that.)

Signed-off-by: Paul Eggleton <paul.eggleton@linux.intel.com>
Signed-off-by: Saul Wold <sgw@linux.intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>