virtio-serial-bus: use bh for unthrottling

virtio-serial-bus: use bh for unthrottling

Instead of calling flush_queued_data when unthrottling, schedule
a bh. That way we can return immediately to the caller, and the
flush uses the same call path as a have_data for callbackee.

No migration change is required because bh are called from vm_stop.

Signed-off-by: Alon Levy <[email protected]>
Signed-off-by: Amit Shah <[email protected]>