spice: fix multihead support

spice: fix multihead support

This patch fixes spice display initialization to handle
multihead properly.

spice-core now keeps track of which QemuConsole has a spice
display channel attached to it and which has not.  It also
manages display channel ids.

spice-display looks at all QemuConsoles and will pick up any
graphic console not yet bound to a spice channel (which in practice
are all non-qxl graphic devices).

Result is that
 (a) you'll get a spice client window for each graphical device
     now (first only without this patch), and
 (b) mixing qxl and non-qxl vga cards works properly.

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>