xenapi: simplify _migrate_disk_resizing_up

Previously it called get_vdi_for_vm_safely but there was no need,
because we already get all the VDI information we need from
snapshot_attached_here.

This will make it easer to expand the method to include the migration of
ephemeral disks.

Part of blueprint xenapi-migrate-ephemeral-disks
Change-Id: I4e0dd1a875ee7e194d38016ff49296a360627aea