Add support for offline migration

Add support for offline migration

Offline migration transfers inactive definition of a domain (which may
or may not be active). After successful completion, the domain remains
in its current state on source host and is defined but inactive on
destination host. It's a bit more clever than virDomainGetXMLDesc() on
source host followed by virDomainDefineXML() on destination host, as
offline migration will run pre-migration hook to update the domain XML
on destination host. Currently, copying non-shared storage is not
supported during offline migration.

Offline migration can be requested with a new migration flag called
VIR_MIGRATE_OFFLINE (which has to be combined with
VIR_MIGRATE_PERSIST_DEST flag).