'sed -i' is not defined in POSIX. It doesn't work on Mac OS X the way
it's used in configure (without suffix argument). This patch implements
Peter Maydell's idea of xattr.h detection.

Cc: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: NPeter Maydell <peter.maydell@linaro.org>
Signed-off-by: NPavel Borzenkov <pavel.borzenkov@gmail.com>
Signed-off-by: NAnthony Liguori <aliguori@us.ibm.com>