Fix OS-DCF:diskconfig XML handling

Removes string to bool conversion for the OS-DCF:diskconfig
attribute during XML deserialising as the diskconfig extension
expects a string (MANUAL or AUTO) not a bool.

Fixes bug 1152439

Change-Id: Id9a5c2559a2891c2a21c437c9a5a534b9885c9bf
Merge "Fix OS-DCF:diskconfig XML handling"