UPSTREAM: VSOCK: transport-specific vsock_transport functions

struct vsock_transport contains function pointers called by AF_VSOCK
core code.  The transport may want its own transport-specific function
pointers and they can be added after struct vsock_transport.

Allow the transport to fetch vsock_transport.  It can downcast it to
access transport-specific function pointers.

The virtio transport will use this.

BUG=chromium:763970
TEST=build with USE=kvm_host on caroline

Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
(cherry picked from commit 0b01aeb3d2fbf16787f0c9629f4ca52ae792f732)
Signed-off-by: Sonny Rao <sonnyrao@chromium.org>

Change-Id: I2c11b52a466140b3c64e2a1ea9025a1b63dcf5d0
Reviewed-on: https://chromium-review.googlesource.com/1058933
Commit-Ready: Sonny Rao <sonnyrao@chromium.org>
Tested-by: Sonny Rao <sonnyrao@chromium.org>
Reviewed-by: Guenter Roeck <groeck@chromium.org>
2 files changed