configure: Do not use 'sed -i' '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 <[email protected]> Reviewed-by: Peter Maydell <[email protected]> Signed-off-by: Pavel Borzenkov <[email protected]> Signed-off-by: Anthony Liguori <[email protected]>