[FIX] web: BasicModel: correctly eval x2manys in domains

When x2many fields have to be evaluated in domains (e.g. ['id',
'in', some_x2many_field]), they must be evaluated as the list of
ids in the relation (whereas in contexts, they are evaluated as a
list of commands). Before this rev., they were always evaluated as
a list of commands.

Note that this didn't work neither before the new views.