[virtio-serial] bz#823818: add a missing feature bit.
[virtio-serial] bz#823818: add a missing feature bit.

Commit f5e27790 rewrote the add_buf usage using the indirect buffers
feature. Problem was that the guest didn't notify the device about
this change.

GoogleCloudPlatform/compute-windows-drivers,daynix/kvm-guest-drivers-windows,YanVugenfirer/virtio-win-arm,virtio-win/kvm-guest-drivers-windows,daynix/kvm-guest-drivers-windows,YanVugenfirer/virtio-win-arm,gnif/kvm-guest-drivers-windows,ladipro/kvm-guest-drivers-windows,YanVugenfirer/virtio-win-arm,ladipro/kvm-guest-drivers-windows,virtio-win/kvm-guest-drivers-windows,YanVugenfirer/virtio-win-arm,YanVugenfirer/kvm-guest-drivers-windows,GoogleCloudPlatform/compute-windows-drivers,YanVugenfirer/kvm-guest-drivers-windows,vrozenfe/kvm-guest-drivers-windows,GoogleCloudPlatform/compute-windows-drivers,YanVugenfirer/kvm-guest-drivers-windows,ladipro/kvm-guest-drivers-windows,virtio-win/kvm-guest-drivers-windows,virtio-win/kvm-guest-drivers-windows,virtio-win/kvm-guest-drivers-windows,YanVugenfirer/kvm-guest-drivers-windows,gnif/kvm-guest-drivers-windows,gnif/kvm-guest-drivers-windows,GoogleCloudPlatform/compute-windows-drivers,ladipro/kvm-guest-drivers-windows,vrozenfe/kvm-guest-drivers-windows,daynix/kvm-guest-drivers-windows,YanVugenfirer/kvm-guest-drivers-windows,daynix/kvm-guest-drivers-windows,vrozenfe/kvm-guest-drivers-windows,vrozenfe/kvm-guest-drivers-windows,vrozenfe/kvm-guest-drivers-windows,gnif/kvm-guest-drivers-windows,daynix/kvm-guest-drivers-windows