uvtd: add VT subsystem

uvtd: add VT subsystem

The VT subsystem manages the virtual VTs and registers them with the
seat/session-scheduler. They manage control and legacy nodes as they are
mostly the same.

The different contexts create these VTs and assign them to cdev clients.
This allows us to split the VT and cdev logic apart so they can be
assigned freely to different clients.

Signed-off-by: David Herrmann <dh.herrmann@gmail.com>