migration: centralize call to migrate_fd_error()

The call to migrate_fd_error() was missing for non-socket backends, so
centralize it in qmp_migrate().

Before:

    (qemu) migrate fd:ffff
    migrate: An undefined error has occurred
    (qemu) info migrate
    (qemu)

After:

    (qemu) migrate fd:ffff
    migrate: An undefined error has occurred
    (qemu) info migrate
    capabilities: xbzrle: off
    Migration status: failed
    total time: 0 milliseconds

(The awful error message will be fixed later in the series).
Reviewed-by: NLuiz Capitulino <lcapitulino@redhat.com>
Signed-off-by: NPaolo Bonzini <pbonzini@redhat.com>

migration-tcp.c

@@ -79,7 +79,6 @@ int tcp_start_outgoing_migration(MigrationState *s, const char *host_port,
 
     s->fd = inet_nonblocking_connect(host_port, tcp_wait_for_connect, s, &local_err);
     if (local_err != NULL) {
-        migrate_fd_error(s);
         error_propagate(errp, local_err);
         return -1;
     }

migration-unix.c

@@ -111,7 +111,6 @@ int unix_start_outgoing_migration(MigrationState *s, const char *path)
 
     if (ret < 0) {
         DPRINTF("connect failed\n");
-        migrate_fd_error(s);
         return ret;
     }
     migrate_fd_connect(s);

migration.c

@@ -524,6 +524,7 @@ void qmp_migrate(const char *uri, bool has_blk, bool blk,
     }
 
     if (ret < 0 || local_err) {
+        migrate_fd_error(s);
         if (!local_err) {
             error_set_errno(errp, -ret, QERR_UNDEFINED_ERROR);
         } else {