ui/vc: move text fields to QemuTextConsole

Now we can instantiate the specific console with its own fields. Pass
the most appropriate type to the various functions, and cast up to
QEMU_CONSOLE as necessary.

Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
Message-Id: <20230830093843.3531473-29-marcandre.lureau@redhat.com>