El host Xen 4.1 (dom0) con discos blktap ("tap: aio:") no se conecta

Problema al usar blktap con xen-4.1 ejecutando Ubuntu Precise stock kernel con dom0 xen-4.1. Yo obtengo: [ 5.580106] XENBUS: Waiting for devices to initialise: 295s...290s. ... [ 300.580288] XENBUS: Timeout connecting to device: device/vbd/51713 (local state 3, remote state 1) Y algunas líneas...