core: missing call to ProceedProxyReturnValue

Added missing call to ProceedProxyReturnValue
in VmReplicateDiskStartVDSCommand and
VmReplicateDiskFinishVDSCommand.

Change-Id: I4ed1b66173cd7a644fc1b412c4c28010f92f0fa8
Bug-Url: https://bugzilla.redhat.com/923194
Signed-off-by: Daniel Erez <derez@redhat.com>