VMware: manage_existing for VMDK driver

VMware: manage_existing for VMDK driver

This patch implements manage_existing for the VMDK driver. Manage
existing creates a volume backing from the vmdk identified by the
source-name option. The source-name format is:
                                    "vmdk_path@vm_inventory_path".

This patch detaches the disk device at vmdk_path from the VM at
vCenter inventory path given by vm_inventory_path and attaches it
to the volume backing.

Implements: blueprint vmdk-manage-existing
Change-Id: I03501bb9bcc9932211174d5a00511af347befd96