spice: fix display initialization

Spice has two display interface implementations:  One integrated into
the qxl graphics card, and one generic which can operate with every
qemu-emulated graphics card.

The generic one is activated in case spice is used without qxl.  The
logic for that only caught the "-vga qxl" case, "-device qxl-vga" goes
unnoticed.  Fix that by adding a check in the spice interface
registration so we'll notice the qxl card no matter how it is created.

https://bugzilla.redhat.com/show_bug.cgi?id=981094

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