Rework loadvm path for subloops

Postcopy needs to have two migration streams loading concurrently;
one from memory (with the device state) and the other from the fd
with the memory transactions.

Split the core of qemu_loadvm_state out so we can use it for both.

Allow the inner loadvm loop to quit and cause the parent loops to
exit as well.

Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
Reviewed-by: Juan Quintela <quintela@redhat.com>
Reviewed-by: Amit Shah <amit.shah@redhat.com>
Signed-off-by: Juan Quintela <quintela@redhat.com>

include/migration/migration.h

@@ -57,6 +57,12 @@ typedef QLIST_HEAD(, LoadStateEntry) LoadStateEntry_Head;
 struct MigrationIncomingState {
     QEMUFile *from_src_file;
 
+    /*
+     * Free at the start of the main state load, set as the main thread finishes
+     * loading state.
+     */
+    QemuEvent main_thread_load_event;
+
     QEMUFile *to_src_file;
     QemuMutex rp_mutex;    /* We send replies from multiple threads */