When listing all devices (geoms) from the given class, skip geoms without

providers.
This prevents from listing geoms like <name>.sync which can be confusing.
It still allows to show details about it by giving its name when listing.

MFC after:	1 week

sbin/geom/core/geom.c

@@ -736,6 +736,8 @@ std_list(struct gctl_req *req, unsigned flags __unused)
 		}
 	} else {
 		LIST_FOREACH(gp, &classp->lg_geom, lg_geom) {
+			if (LIST_EMPTY(&gp->lg_provider))
+				continue;
 			show_one_geom(gp);
 		}
 	}