util: introduce MIN_NON_ZERO

at least in block layer we have the case of limits being defined for a
BlockDriverState. However, in this context often zero (0) has the special
meanining of undefined which means no limit. If two of those limits are
combined and the minimum is needed the minimum function should only return
zero if both parameters are zero.
Signed-off-by: NPeter Lieven <pl@kamp.de>
Reviewed-by: NMax Reitz <mreitz@redhat.com>
Signed-off-by: NStefan Hajnoczi <stefanha@redhat.com>

include/qemu/osdep.h

@@ -68,6 +68,12 @@ typedef signed int              int_fast16_t;
 #define MAX(a, b) (((a) > (b)) ? (a) : (b))
 #endif
 
+/* Minimum function that returns zero only iff both values are zero.
+ * Intended for use with unsigned values only. */
+#ifndef MIN_NON_ZERO
+#define MIN_NON_ZERO(a, b) (((a) != 0 && (a) < (b)) ? (a) : (b))
+#endif
+
 #ifndef ROUND_UP
 #define ROUND_UP(n,d) (((n) + (d) - 1) & -(d))
 #endif