Stop playing games accessing struct fields directly from go and call the
supported libvirt APIs instead. This makes the code clearer and safer
from Go.

The string list code still needs direct pointer games, since libvirt
does not expose any virTypedParamsGetStringList() method, as none of its
APIs actually require this feature yet. The Go binding still wants this
impl at least for the test suite in order to validate the packing of
string lists.

Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>