qemu: migration: new migration param for persistent destination XML

qemu: migration: new migration param for persistent destination XML

Migration API allows to specify a destination domain configuration.
Offline domain has only inactive XML and it is replaced by configuration
specified using VIR_MIGRATE_PARAM_DEST_XML param. In case of live
migration VIR_MIGRATE_PARAM_DEST_XML param is applied for active XML.

This commit introduces the new VIR_MIGRATE_PARAM_PERSIST_XML param
that can be used within live migration to replace persistent/inactive
configuration.

Required for: https://bugzilla.redhat.com/show_bug.cgi?id=835300