Future spice-server versions will call the client_monitors_config
callback with the monitors list filtered to only include the monitors
of the given display channel (aka QXLInstance).  Luckily this is easily
detectable at runtime, so we can prepare for that in advance and also
make qemu compatible with both old and new spice-server versions.

While being at it also use the console index instead of head number as
array index.  The later doesn't work correctly in case multiple display
devices are present.

Cc: spice-devel@lists.freedesktop.org
Signed-off-by: NGerd Hoffmann <kraxel@redhat.com>
Reviewed-by: NLukáš Hrázký <lhrazky@redhat.com>
Message-id: 20181012114551.28809-1-kraxel@redhat.com