When building qemu-kvm for openSUSE:Factory, I am getting a
warning in the pipe2 detection performed by configure, which
prevents using --enable-werror.

Change detection code to use return value of pipe2.
Signed-off-by: NBruce Rogers <brogers@suse.com>
Reviewed-by: NPeter Maydell <peter.maydell@linaro.org>
Signed-off-by: NBlue Swirl <blauwirbel@gmail.com>