Refuse to block migrate instances with config drive

libvirt does not block migrate read only devices such as config
drives. It decides that the config drive is read only because it
is attached as a cdrom. So, we just throw our hands up in the
air and bravely refuse to block migrate those instances.

Partial-Bug: 1246201

Change-Id: I24f4f83c1ddf1cf3e37db4663ab82e61f0a2d00f
DocImpact: you should read all that stuff above.
Merge "Refuse to block migrate instances with config drive"