libxl: require qemu in dom0 for multiple stubdomain consoles

libxl: require qemu in dom0 for multiple stubdomain consoles

Device model stubdomains (both Mini-OS + qemu-trad and linux + qemu-xen)
are always started with at least 3 consoles: log, save, and restore.
Until xenconsoled learns how to handle multiple consoles, this is needed
for save/restore support.

For Mini-OS stubdoms, this is a bug.  In practice, it works in most
cases because there is something else that triggers qemu in dom0 too:
vfb/vkb added if vnc/sdl/spice is enabled.

Additionally, Linux-based stubdomain waits for all the backends to
initialize during boot. Lack of some console backends results in
stubdomain startup timeout.

This is a temporary patch until xenconsoled will be improved.

Signed-off-by: Marek Marczykowski-Górecki <marmarek@invisiblethingslab.com>
[Updated commit message with Marek's explanation from mailing list.]
Signed-off-by: Jason Andryuk <jandryuk@gmail.com>
Acked-by: Ian Jackson <ian.jackson@eu.citrix.com>