Fix more OS-DCF:diskConfig XML handling

Add handling in the deserialisation code to handle the
OS-DCF:diskConfig attribute in the rebuild and resize actions so
the diskConfig extension can handle it properly. Formerly only
the older auto_disk_config attribute was managed (and incorrectly so).

Fixes bug 1153133

Change-Id: I68479d258cf23083274dd21b1f9eabab2feeb093
Merge "Fix more OS-DCF:diskConfig XML handling"