Revert "qxl: add migration blocker to avoid pre-save assert"

This reverts commit 86dbcdd9.

The pre-save assert is gone now, so the migration blocker
is not needed any more.

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Message-Id: <20210721093347.338536-3-kraxel@redhat.com>