xenapi: refactor resize_up_root_vdi

Some refactoring to move things into vm_utils where possible, and
setting the stage for resizing ephemeral disks.

Needed by blueprint xenapi-resize-ephemeral-disks
Change-Id: Iba26ddb8b3d9041981317c47bb3b53733c004cea
Merge "xenapi: refactor resize_up_root_vdi"