Fix: cpp-common/bt2: more specific assertion in `CommonVariantWithIntegerSelectorFiel...

Fix: cpp-common/bt2: more specific assertion in `CommonVariantWithIntegerSelectorFieldClass` constructor

It seems like we can check that the field class is not only a variant,
but more specifically a variant with integer selector.

Change-Id: Ib062779d957ba1e859247c891027121479ae3adb
Signed-off-by: Simon Marchi <simon.marchi@efficios.com>
Reviewed-on: https://review.lttng.org/c/babeltrace/+/12051
Reviewed-by: Philippe Proulx <eeppeliteloop@gmail.com>