Extensible migration APIs

Extensible migration APIs

This patch introduces two new APIs virDomainMigrate3 and
virDomainMigrateToURI3 that may be used in place of their older
variants. These new APIs take optional migration parameters (such as
bandwidth, domain XML, ...) in an array of virTypedParameters, which
makes adding new parameters easier as there's no need to introduce new
APIs whenever a new migration parameter needs to be added. Both APIs are
backward compatible and will automatically use older migration calls in
case the new calls are not supported as long as the typed parameters
array does not contain any parameter which was not supported by the
older calls.