lib: bt_graph_create(): accept MIP version

lib: bt_graph_create(): accept MIP version

This patch makes bt_graph_create() accept the message interchange
protocol (MIP) version with which to configure the graph.

This MIP version will usually come from
bt_get_greatest_operative_mip_version() or bt_get_maximal_mip_version().

The components can access the configured MIP version with
bt_self_component_get_graph_mip_version().

As of this patch the only possible MIP version is 0. bt_graph_create()
does not accept a MIP version that is greater than
bt_get_maximal_mip_version() because almost all the library functions
need to know the configured MIP version to behave accordingly, should
the semantics change in the future.

In Python, Graph.__init__() accepts a `mip_version` parameter which
defaults to 0. A Python component can get the configured MIP version of
its operating graph with the `_UserComponent._graph_mip_version`
property.

All the sites where bt_graph_create() is called pass 0 to it. This is
correct for tests as we always control the component classes and
initialization parameters, so we know that 0 is always supported by
them. The locations where bt_get_greatest_operative_mip_version() must
be used are the CLI and `bt2.TraceCollectionMessageIterator`. Those
sites will be addressed by subsequent patches.

Signed-off-by: Philippe Proulx <eeppeliteloop@gmail.com>
Change-Id: I324a317922a8caa5557043ae02b97e58fc91d92d
Reviewed-on: https://review.lttng.org/c/babeltrace/+/1878
Tested-by: jenkins <jenkins@lttng.org>
Reviewed-by: Simon Marchi <simon.marchi@efficios.com>