migration: Fix fd protocol for incoming defer

Currently, incoming migration through fd supports only command-line case:
E.g.
    fork();
    fd = open();
    exec("qemu ... -incoming fd:%d", fd);

It's possible to use add-fd commands to pass fd for migration, but it's
invalid case. add-fd works with fdset but not with particular fds.

To work with getfd in incoming defer it's enough to use monitor_fd_param
instead of strtol. monitor_fd_param supports both cases:
* fd:123
* fd:fd_name (added by getfd).

And also the use of monitor_fd_param improves error messages.

Signed-off-by: Yury Kotov <yury-kotov@yandex-team.ru>
Reviewed-by: Peter Xu <peterx@redhat.com>
Reviewed-by: Juan Quintela <quintela@redhat.com>
Signed-off-by: Juan Quintela <quintela@redhat.com>

migration/fd.c

@@ -52,12 +52,14 @@ static gboolean fd_accept_incoming_migration(QIOChannel *ioc,
     return G_SOURCE_REMOVE;
 }
 
-void fd_start_incoming_migration(const char *infd, Error **errp)
+void fd_start_incoming_migration(const char *fdname, Error **errp)
 {
     QIOChannel *ioc;
-    int fd;
+    int fd = monitor_fd_param(cur_mon, fdname, errp);
+    if (fd == -1) {
+        return;
+    }
 
-    fd = strtol(infd, NULL, 0);
     trace_migration_fd_incoming(fd);
 
     ioc = qio_channel_new_fd(fd, errp);

migration/fd.h

@@ -16,7 +16,7 @@
 
 #ifndef QEMU_MIGRATION_FD_H
 #define QEMU_MIGRATION_FD_H
-void fd_start_incoming_migration(const char *path, Error **errp);
+void fd_start_incoming_migration(const char *fdname, Error **errp);
 
 void fd_start_outgoing_migration(MigrationState *s, const char *fdname,
                                  Error **errp);