Pass config file object through to driver open methods

Pass config file object through to driver open methods

The virConnectOpenInternal method opens the libvirt client
config file and uses it to resolve things like URI aliases.

There may be driver specific things that are useful to
store in the config file too, so rather than have them
re-parse the same file, pass the virConfPtr down to the
drivers.

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