[PATCH] allow mounting of the hdiso source (#498591)

[Chris Lumens <clumens redhat com>]

I really, REALLY despise the name "deviceMostlyImmutable", but I couldn't come
up with a better name that doesn't sound confusing.  The various protected
properties and methods we have only make this harder.  Anyway, I have not
tested this patch but I feel good about it.  If everyone likes this direction
I can put together an updates image for the reporter.

- Chris