configure: fix detection for xattr.h on modern distributions

Modern distributions place xattr.h in /usr/include/sys, and fold
libattr.so into libc.  They also don't have an ENOATTR.

Make configure detect this, and add a qemu-xattr.h file that
directs the #include to the right place.

Signed-off-by: Avi Kivity <avi@redhat.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>
diff --git a/linux-user/syscall.c b/linux-user/syscall.c
index 9f5da36..f227097 100644
--- a/linux-user/syscall.c
+++ b/linux-user/syscall.c
@@ -71,7 +71,7 @@
 #include <sys/epoll.h>
 #endif
 #ifdef CONFIG_ATTR
-#include <attr/xattr.h>
+#include "qemu-xattr.h"
 #endif
 
 #define termios host_termios