conf: gfx: Add egl-headless as a member to virDomainGraphicsDef struct

Since we need to specify the rendernode option onto QEMU cmdline, we
need this union member to retain consistency in how we build the
cmdline.
Signed-off-by: NErik Skultety <eskultet@redhat.com>
Reviewed-by: NJán Tomko <jtomko@redhat.com>

src/conf/domain_conf.h

@@ -1659,6 +1659,9 @@ struct _virDomainGraphicsDef {
             virTristateBool gl;
             char *rendernode;
         } spice;
+        struct {
+            char *rendernode;
+        } egl_headless;
     } data;
     /* nListens, listens, and *port are only useful if type is vnc,
      * rdp, or spice. They've been extracted from the union only to