maint: avoid 'const fooPtr' in python bindings

maint: avoid 'const fooPtr' in python bindings

'const fooPtr' is the same as 'foo * const' (the pointer won't
change, but it's contents can).  But in general, if an interface
is trying to be const-correct, it should be using 'const foo *'
(the pointer is to data that can't be changed).

Fix up offenders in the python bindings.

* python/generator.py (py_types): Drop useless conversions.
* python/libvirt-override.c (getPyVirTypedParameter)
(setPyVirTypedParameter): Use intended type.

Signed-off-by: Eric Blake <eblake@redhat.com>