cli: convert: allow multiple paths to be passed as leftover arguments

cli: convert: allow multiple paths to be passed as leftover arguments

Allow this:

    babeltrace /path/to/trace1 /path/to/trace2 /path/to/trace3

This worked with Babeltrace 1 and needs to work with Babeltrace 2.

What's done here is that a _base_ implicit set of arguments for an
eventual source.ctf.fs component is created implicitly or by default
like it used to be. For each leftover argument, we copy this base set of
arguments and append the `path` parameter with the specific leftover
argument. Then we name and connect those source components as usual.
What's left is one source.ctf.fs component for each individual path.
Each individual component can still recurse.

tests/cli/test_convert_args is updated because it used to check that a
command line with multiple leftovers was expected to fail, but it
doesn't with this patch.

Signed-off-by: Philippe Proulx <eeppeliteloop@gmail.com>
Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>