libxl: support VIR_MIGRATE_PERSIST_DEST migration flag
By default, virt-manager (and likely other libvirt-based apps) sets
the VIR_MIGRATE_PERSIST_DEST flag when invoking the migrate API, which
fails in a Xen setup since the libxl driver does not support the flag.
Persisting a domain is a trivial task in the grand scheme of migration,
so be nice to libvirt apps and add support for VIR_MIGRATE_PERSIST_DEST
in the libxl driver.