validate specified volumes to boot from at the API layer

This causes the create to fail fast instead of putting
the instance into error later in the process.

Related to bug: 1069904
Change-Id: I5f7c8d20d3ebf33ce1ce64bf0a8418bd2b5a6411
Merge "validate specified volumes to boot from at the API layer"
Merge remote-tracking branch 'citrix/master' into build-temp