Add QEMU guest support

* Build the virtio_mmio, virtio_net, virtio_scsi, and virtio_console
  drivers by default in place of HTIF.
* Retain support for QEMU's emulated 8250 UART until it is determined
  how to use vport0p1 as the primary console terminal.
* Explicitly instantiate virtio MMIO transports through boot parameters.
* Clear the corresponding SR_IP bit when acknowledging IRQs, as expected
  by the simulator implementation.
* Avoid clobbering the SR_IP field when returning from exceptions.