Fix: use channel per domain default values

This is related to bug497 but does not fix it. Using the command line and the
lttng_channel_set_default_attr() API call, the default read and switch timer
were not correct for UST and Kernel domain since we now use specific default
values for each of them.

Reported-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Signed-off-by: David Goulet <dgoulet@efficios.com>