xenconfig: fix xml to xl.cfg conversion with no graphics

If no graphics element is in XML xenFormatXLSpice will access
graphics without checking it has one in the first place, leading to a
segmentation fault.
Signed-off-by: NJoao Martins <joao.m.martins@oracle.com>

src/xenconfig/xen_xl.c

@@ -1168,7 +1168,7 @@ xenFormatXLSpice(virConfPtr conf, virDomainDefPtr def)
     virDomainGraphicsListenDefPtr glisten;
     virDomainGraphicsDefPtr graphics;
 
-    if (def->os.type == VIR_DOMAIN_OSTYPE_HVM) {
+    if (def->os.type == VIR_DOMAIN_OSTYPE_HVM && def->graphics) {
         graphics = def->graphics[0];
 
         if (graphics->type == VIR_DOMAIN_GRAPHICS_TYPE_SPICE) {