CLOUDSTACK-4470, findHostsForMigration api isn't setting the requiresStorageMotion flag to true
when a virtual machine is to be migrated across cluster and the source and destination host do
not share storage pools. migrationRequired flag was introduced in a recent commit that was always
set to false for XenServer. This caused the destination host to be flaged as not requiring
storage motion. Fixed the scope of the boolean and defaulted it to true. Other checks validate
if storage motion is required for XenServer.