migration: yay, buffering is gone

Buffering was needed because blocking writes could take a long time
and starve other threads seeking to grab the big QEMU mutex.

Now that all writes (except within _complete callbacks) are done
outside the big QEMU mutex, we do not need buffering at all.

Reviewed-by: Orit Wasserman <owasserm@redhat.com>
Reviewed-by: Juan Quintela <quintela@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Juan Quintela <quintela@redhat.com>

include/migration/migration.h

@@ -34,9 +34,6 @@ struct MigrationState
     int64_t bandwidth_limit;
     size_t bytes_xfer;
     size_t xfer_limit;
-    uint8_t *buffer;
-    size_t buffer_size;
-    size_t buffer_capacity;
     QemuThread thread;
     QEMUBH *cleanup_bh;