[virtcon] Convert to v2 component

Convert virtcon to a v2 component.

This should only have a small timing change.
Virtcon used to always be launched by the driver_manager, and
now it has become a component that is not launched eagerly.
For bringup/zedboot builds, virtcon is launched when ConsoleLauncher
sends it shells. For core builds, virtcon is launched when
appmgr sends it shells.

Fixed: 61528
TEST: Paved astro, zedbooted astro
      Ran all virtcon tests.
Change-Id: I451d15add2209fb7d810dbf80dfac82ba768c816
Reviewed-on: https://fuchsia-review.googlesource.com/c/fuchsia/+/448236
Commit-Queue: David Gilhooley <dgilhooley@google.com>
Reviewed-by: Drew Fisher <zarvox@google.com>
Reviewed-by: Suraj Malhotra <surajmalhotra@google.com>
Testability-Review: Suraj Malhotra <surajmalhotra@google.com>