Xenapi: Allow volume backed instances to migrate

The migrate process had no provisions for a non image backed instance.
This remedies that by doing a few things:

Don't snapshot the root disk on the source host.
Don't try to import the root disk on the destination host.
Connect the volume to the destination host as the boot device.

Change-Id: Ia453cf33a36803b1b5447c6240988a3dd603f328
Closes-bug: #1375480
VMware: Remove unused _get_orig_vm_name_label from vmops

Change-Id: Ide8653e455b96e0946eae50c808b717793312c8f
VMware: Remove unused _check_if_folder_file_exists from vmops

Change-Id: I7aa49ba00c61257275f0200bb39da0161a2fab7b
Merge "Xenapi: Allow volume backed instances to migrate"
Merge "VMware: Remove unused _get_orig_vm_name_label from vmops"
Merge "VMware: Remove unused _check_if_folder_file_exists from vmops"