default devices: virtio consoles.

default devices: virtio consoles.

This patch adds a variable default_virtcon which says whenever a default
virtio console should be added.  It is disabled by default, followup
patch will enable it for s390.  It is cleared when qemu finds
'-virtiocon', '-device virtio-console-s390' or '-device
virtio-console-pci' on the command line.

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>
(cherry picked from commit aee1b935c53cc747528138a85fcce8163d272598)