vhost-vsock: handle common features in vhost-vsock-common

vhost-vsock: handle common features in vhost-vsock-common

virtio-vsock features, like VIRTIO_VSOCK_F_SEQPACKET, can be handled
by vhost-vsock-common parent class. In this way, we can reuse the
same code for all virtio-vsock backends (i.e. vhost-vsock,
vhost-user-vsock).

Let's move `seqpacket` property to vhost-vsock-common class, add
vhost_vsock_common_get_features() used by children, and disable
`seqpacket` for vhost-user-vsock device for machine types < 6.2.

The behavior of vhost-vsock device doesn't change; vhost-user-vsock
device now supports `seqpacket` property.

Signed-off-by: Stefano Garzarella <sgarzare@redhat.com>
Message-Id: <20210921161642.206461-3-sgarzare@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>