vmm: migration: add vm.migration-receive-ready event

The new event allows management software to handle the migration better
via events. The `vm.migration-receive-ready` event tells that the VMM is
ready to accept connections whereas `vm.migration-receive-started` means
a migration is incoming.

On-behalf-of: SAP philipp.schuster@sap.com
Signed-off-by: Philipp Schuster <philipp.schuster@cyberus-technology.de>

vmm/src/lib.rs

@@ -2534,6 +2534,9 @@ impl RequestHandler for Vmm {
 
         let mut listener =
             migration_transport::receive_migration_listener(&receive_data_migration.receiver_url)?;
+
+        event!("vm", "migration-receive-ready");
+
         // Accept the connection and get the socket
         let mut socket = listener.accept()?;