CLOUDSTACK-5560: Reattach of data disk fails for hyperv. When a data disk
is attached a hard disk drive is created on the scsi controller. On detach
the data disk is removed from the drive but the disk drive is left behind.
On reattach the agent was again trying to create a disk drive while it was
already present. Fixed the agent code to look up for disk drive while
attaching and if one is not found then only to create the drive for
attaching a data disk.