raw-posix: inline paio_ioctl into hdev_aio_ioctl

clang now warns about an unused function:
  CC    block/raw-posix.o
block/raw-posix.c:707:26: warning: unused function paio_ioctl
[-Wunused-function]
static BlockDriverAIOCB *paio_ioctl(BlockDriverState *bs, int fd,
                         ^
1 warning generated.

because the only use of paio_ioctl() is inside a #if defined(__linux__)
guard and it is static now.

Reported-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
