migration: block-dirty-bitmap: add missing qemu_mutex_lock_iothread

migration: block-dirty-bitmap: add missing qemu_mutex_lock_iothread

init_dirty_bitmap_migration assumes the iothread lock (BQL)
to be held, but instead it isn't.

Instead of adding the lock to qemu_savevm_state_setup(),
follow the same pattern as the other ->save_setup callbacks
and lock+unlock inside dirty_bitmap_save_setup().

Signed-off-by: Emanuele Giuseppe Esposito <[email protected]>
Reviewed-by: Stefan Hajnoczi <[email protected]>
Message-Id: <20211005080751.3797161[email protected]>
Signed-off-by: Paolo Bonzini <[email protected]>