After the recent introduction of node panels, the utility that
allowed scripters to connect sockets to or from a virtual socket was
broken. This happens when creating an interface before connecting
sockets. Sockets can have a subtype, while interfaces cannot.
For instance, the `NodeSocketFloatFactor` type cannot be used
directly, it has to be converted to `NodeSocketFloat`.
This commit makes sure that the socket type gets stripped of their
last part, which indicates the subtype, before creating the new
interface.
