core: Enhance RO need to update check

RO property's null is interpreted as false in VmDevice and remains null
in Disk. This may result in a wrong decision whether to an update is in
need or not. This patch addresses this issue.

Change-Id: I9bbfc60c3e9bda0133eec9b1db116f8e7bad9d1d
Related-To: https://bugzilla.redhat.com/1057721
Signed-off-by: Vered Volansky <vvolansk@redhat.com>