vl.c: check for qxl availability

Check for qxl availability in vl.c. This will allow to remove #ifdef
CONFIG_SPICE .. #endif later in this series

Cc: Anthony Liguori <aliguori@us.ibm.com>
Cc: Gerd Hoffmann <kraxel@redhat.com>
Signed-off-by: Aurelien Jarno <aurelien@aurel32.net>
