rpmsg: virtio: add endianness conversions

According to the VirtIO 1.0 spec data, sent over virtual queues must
be in little-endian format. Update the RPMsg VirtIO implementation
to enforce that but let legacy configurations continue use native
endianness.

Acked-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Guennadi Liakhovetski <guennadi.liakhovetski@linux.intel.com>
Reviewed-by: Mathieu Poirier <mathieu.poirier@linaro.org>
Tested-by: Arnaud Pouliquen <arnaud.pouliquen@st.com>
Link: https://lore.kernel.org/r/20200721085638.GA3815@ubuntu


Signed-off-by: Bjorn Andersson <bjorn.andersson@linaro.org>