virtio-mmio: Clear v2 transport state on soft reset

virtio-mmio: Clear v2 transport state on soft reset

At the moment when the guest writes a status of 0, we only reset the
virtio core state but not the virtio-mmio state. The virtio-mmio
specification says (v1.1 cs01, 4.2.2.1 Device Requirements:
MMIO Device Register Layout):

    Upon reset, the device MUST clear all bits in InterruptStatus and
    ready bits in the QueueReady register for all queues in the device.

The core already takes care of InterruptStatus by clearing isr, but we
still need to clear QueueReady.

It would be tempting to clean all registers, but since the specification
doesn't say anything more, guests could rely on the registers keeping
their state across reset. Linux for example, relies on this for
GuestPageSize in the legacy MMIO tranport.

Fixes: 44e687a4d9ab ("virtio-mmio: implement modern (v2) personality (virtio-1)")
Signed-off-by: Jean-Philippe Brucker <jean-philippe@linaro.org>
Message-Id: <20191213095410.1516119-1-jean-philippe@linaro.org>
Reviewed-by: Sergio Lopez <slp@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>