vconsole: match on vtcon events, not fbcon ones

I observe that upon loading of framebuffer drivers, I do not get the
desired system font, but the kernel-level defaults (usually
lib/fonts/font_8x16.c, but your mileage may vary depending on kernel
config and boot options).

The fbcon driver may be loaded at a time way before the first
framebuffer device is active, such that the vconsole setup helper
runs too early.

The existing rule is non-fitting. The going live of the fbcon kernel
component does not indicate the proper time at which to load the
visuals, which really ought to be done when a new vtcon object comes
into existence. (The font table is a per-vtcon property.)

(cherry picked from commit a52750d1)