engine: Cleanup of HotPlugDiskToVmCommand

1. Making class member private instead of protected
2. Replacing deprecated message with more specific one
3. Simplifing CDA

Change-Id: I2cdc4e19da8768af9cd6659763af622b3e630e1d
Signed-off-by: Sergey Gotliv <sgotliv@redhat.com>
Related-to: https://bugzilla.redhat.com/1003649