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 <alevy@redhat.com>
Signed-off-by: Amit Shah <amit.shah@redhat.com>

hw/virtio-serial.h

@@ -119,6 +119,11 @@ struct VirtIOSerialPort {
     uint32_t iov_idx;
     uint64_t iov_offset;
 
+    /*
+     * When unthrottling we use a bottom-half to call flush_queued_data.
+     */
+    QEMUBH *bh;
+
     /* Identify if this is a port that binds with hvc in the guest */
     uint8_t is_console;