Reducing stack frame size in stream_process_mem2s()

Reducing stack frame size in stream_process_mem2s()

This patch allocates memory for txbuf in struct Stream rather than the stack.
As a result, the stack frame size is reduced of stream_process_mem2s().

Signed-off-by: Rutuja Shah <rutu.shah.26@gmail.com>
Reviewed-by: Edgar E. Iglesias <edgar.iglesias@xilinx.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Reviewed-by: Alistair Francis <alistair.francis@xilinx.com>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>