VMware: manage_existing_get_size for VMDK driver

VMware: manage_existing_get_size for VMDK driver

This patch implements manage_existing_get_size 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 checks for the existence of a disk device at
path vmdk_path and attached to the VM at vCenter inventory
path given by vm_inventory_path. If the disk device exists
it returns its size.

Implements: blueprint vmdk-manage-existing
Change-Id: Id5698c2452aed454f22ac68fee8dd90046d858e1