Fix: Enable event after start command

Refer to the bug tracker for more information on this bug. In a
nutshell, it was not possible to enable channel/event after a start
command has been issued (for both kernel and UST).

Note that it is still NOT possible to enable a channel after a start but
the use case here was the following:

$ lttng create
$ lttng start
$ lttng enable-event -a -u (or -k)

The first starts does NOT create a kernel or UST session so the
following enable event creates the session for the given domain but
failed to start the session after.

https://bugs.lttng.org/issues/346
https://bugs.lttng.org/issues/394

Fixes #346 #394

Signed-off-by: David Goulet <dgoulet@efficios.com>