[ACS42] Cherry-pick for CLOUDSTACK-4423 (93b40b0ca6a796ffbacd80c40897f6ada6284bc4)

[Wido den Hollander]

Hi,

Can you cherry-pick 93b40b0ca6a796ffbacd80c40897f6ada6284bc4 into 4.2?
It fixes CLOUDSTACK-4423 and enables RBD volumes to be migrated to other 
primary storages.

Thank you!

Wido