If a display is backed by a specialized VC, allow to override the
default "vc:80Cx24C".

As suggested by Paolo, if the display doesn't implement a VC (get_vc()
returns NULL), use a fallback that will use a muxed console on stdio.

This changes the behaviour of "qemu -display none", to create a muxed
serial/monitor by default (on TTY & not daemonized).

Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Reviewed-by: Thomas Huth <thuth@redhat.com>