sessiond: add variant selector intervals

sessiond: add variant selector intervals

In CTF 2, variant options are chosen using a set of ranges. This differs
from CTF 1.8 (and the current API) which assumed that a variant had
options that perfectly matched the selector enumeration's mappings.

Add integer mappings to variants and also carry a choice name that
becomes the name used by the TSDL producer.

Change-Id: Ib31cf9cc211dad36759bd73dc722c49d634c666b
Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>