qapi: Smooth another visitor error checking pattern

Convert

    visit_type_FOO(v, ..., &ptr, &err);
    ...
    if (err) {
        ...
    }

to

    visit_type_FOO(v, ..., &ptr, errp);
    ...
    if (!ptr) {
        ...
    }

for functions that set @ptr to non-null / null on success / error.

Eliminate error_propagate() that are now unnecessary.  Delete @err
that are now unused.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Message-Id: <20200707160613.848843-40-armbru@redhat.com>
diff --git a/hw/core/numa.c b/hw/core/numa.c
index 1b3267a..d1a94a1 100644
--- a/hw/core/numa.c
+++ b/hw/core/numa.c
@@ -516,10 +516,10 @@
     Error *err = NULL;
     Visitor *v = opts_visitor_new(opts);
 
-    visit_type_NumaOptions(v, NULL, &object, &err);
+    visit_type_NumaOptions(v, NULL, &object, errp);
     visit_free(v);
-    if (err) {
-        goto end;
+    if (!object) {
+        return -1;
     }
 
     /* Fix up legacy suffix-less format */
@@ -530,7 +530,6 @@
 
     set_numa_options(ms, object, &err);
 
-end:
     qapi_free_NumaOptions(object);
     if (err) {
         error_propagate(errp, err);