libqos: add missing virtio-9p feature negotiation

libqos: add missing virtio-9p feature negotiation

VIRTIO Device Initialization requires feature negotiation.  The libqos
virtio-9p driver lacks feature negotiation and is therefore
non-compliant.

libqos tests acknowledge all feature bits advertised by the device,
except VIRTIO_F_BAD_FEATURE (which devices use to detect broken
drivers!) and VIRTIO_RING_F_EVENT_IDX (which is not implemented in
libqos and accepting it would break notifications).

Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Message-Id: <20191023100425.12168-7-stefanha@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Reviewed-by: Thomas Huth" <thuth@redhat.com>