bug in old versions of QEMU (and Xen, and other places using QEMU code).
On those buggy emulated UARTs, the "TX idle" interrupt gets lost; with
this workaround, we spinwait for the TX to happen and then send ourselves
the interrupt.  It's ugly but it works, while minimizing the impact on
the code for the !broken_txfifo case.

MFC after:	2 weeks