session: jump to VT up front for X sessions

It turns out that in order for X to work without being
run as root, it needs to have its VT be active at start up.
This is because it bails out of some of its logind codepaths
if it gets given a "paused" device by logind, and then sometimes
fails in its fallback code paths as it races to open
/dev/dri/card0 while logind is still giving the user
permissions to it.

As a workaround, until X is fixed, this commit jumps to the VT up
front before starting X. That adds flicker, which we'll address in
a follow up commit.

https://bugzilla.gnome.org/show_bug.cgi?id=745031