vdsm: Refactoring the device dict creation

The device dictionaries are created on multiple different locations and
explicitly.
To avoid this duplication we're now moving it to one location where we
define an immutable tuple of tuples and generate the dictionaries from
there.

Change-Id: I4ce2afa73f1f8d1aea15782fec7fee52ee49d66e
Signed-off-by: Vinzenz Feenstra <vfeenstr@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/19733
Reviewed-by: Dan Kenigsberg <danken@redhat.com>