qemu: Replace workaround with proper usermode fix for shmat

We were using a workaround to avoid problems with the behaviour of shmat()
calls in usermode qemu. Switch to patches from upstream which are in review
but not merged yet.

Update the mmap fixed/noreplace workaround for the changes.

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>