We often lookup the private data and retrieve it via NM_DEVICE_GET_PRIVATE(),
which in turn calls G_TYPE_INSTANCE_GET_PRIVATE().

Instead cache the pointer to the private data.

There are up- and downsides:

 - requries additional sizeof(gpointer) bytes for each NMDevice.
 + retrieving the private pointer will be slightly faster.
 + easier debugging in gdb as it is currently often a pain to
   retrieve the private data.

But most importantly, the allows to change our common pattern
to first cache the private data in a variable @priv. That is
often cumbersome to write, especially for short functions.
This change gives us a choice to use self->priv directly.

Such a change should not be aimed for every class. Instead it makes
mostly sense for NMDevice, where it pays off better due to the
class' size and ubiquitous use.

https://mail.gnome.org/archives/networkmanager-list/2015-December/msg00017.html