QemuOpts: Convert qemu_opts_foreach() to Error

Retain the function value for now, to permit selective conversion of
its callers.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Acked-by: Kevin Wolf <kwolf@redhat.com>
diff --git a/net/vhost-user.c b/net/vhost-user.c
index f1a15ce..cce168a 100644
--- a/net/vhost-user.c
+++ b/net/vhost-user.c
@@ -204,7 +204,7 @@
     return chr;
 }
 
-static int net_vhost_check_net(QemuOpts *opts, void *opaque)
+static int net_vhost_check_net(void *opaque, QemuOpts *opts, Error **errp)
 {
     const char *name = opaque;
     const char *driver, *netdev;
@@ -245,7 +245,7 @@
 
     /* verify net frontend */
     if (qemu_opts_foreach(qemu_find_opts("device"), net_vhost_check_net,
-                          (char *)name)) {
+                          (char *)name, NULL)) {
         return -1;
     }