This backwards compatible patch adds the possibility to use new
nm_device_generate_connection() API via update_connection() virtual
method implementations in NMDevice subclasses.

Compatibility is achieved by first trying to use the older API and
match_l2_config() virtual method and only then moving on to
update_connection().

The nm_device_generate_connection() calls update_connection() to create
type-specific NMSetting instances and verifies the connection before
returning it. To avoid tinkering with NMSettingConnection in
update_connection() we use a class attribute called connection_type
which is used by nm_device_generate_connection() itself.

Known issues:

* nm_device_generate_connection() method doesn't implement DHCP lease
configuration matching. We shouldn't actually need it but if a use case
for that will come out, we can fix it later.

* nm_device_generate_connection() doesn't fill in the slave-specific
options.

* update_connection() is not implemented and connection_type is not set
in the subclasses. This will be fixed in individual patches.

* NMSetting's compare_property() implementations in combination with
NM_SETTING_COMPARE_FLAG_CANDIDATE are not yet fully ready thus rendering
false negatives in some cases. Same as above.

Acked-by: Dan Winship <danw@gnome.org>
Acked-by: Thomas Haller <thaller@redhat.com>