libqos: read QVIRTIO_MMIO_VERSION register

libqos: read QVIRTIO_MMIO_VERSION register

There was no real virtio-mmio ABI change between Legacy and VIRTIO 1.0
except that the Version field was incremented from 1 to 2.

However, QEMU does not allow Legacy drivers to perform VIRTIO 1.0
operations like accessing 64-bit feature bits.  Since we will introduce
64-bit feature bit support we need a way to differentiate between
virtio-mmio Version 1 and 2 to avoid upsetting QEMU when we operate in
Legacy mode.

Stash away the Version field so later patches can change behavior
depending on the version.

Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Message-Id: <20191023100425.12168-3-stefanha@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>