migration: cleanup migration (including thread) in the iothread

Perform final cleanup in a bottom half, and add joining the thread to
the series of cleanup actions.

migrate_fd_error remains for connection error, but it doesn't need
to cleanup anything anymore.

Reviewed-by: Orit Wasserman <owasserm@redhat.com>
Reviewed-by: Juan Quintela <quintela@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Juan Quintela <quintela@redhat.com>

include/migration/migration.h

@@ -38,6 +38,7 @@ struct MigrationState
     size_t buffer_size;
     size_t buffer_capacity;
     QemuThread thread;
+    QEMUBH *cleanup_bh;
 
     QEMUFile *file;
     int fd;