ctf: make ctf_msg_iter_seek assert that the seek callback is not NULL

There should be no reason to call ctf_msg_iter_seek if there is no seek
callback.  It's the user of the ctf_msg_iter who passes medium
operations (at creation time), so it should be aware if there is a seek
callback or not.

This allows removing the UNSUPPORTED values in enum ctf_msg_iter_status
and enum ctf_msg_iter_medium_status.

Change-Id: Ic3591b1e909c7cf0e12d18f40b921a180cf344c8
Signed-off-by: Simon Marchi <simon.marchi@efficios.com>