Fix shrinking volumes with the delta flag

This never worked.

In 0.9.10 when this API was introduced, it was intended that
the SHRINK flag combined with DELTA would shrink the volume by
the specified capacity (to avoid passing negative numbers).
See commit 055bbf45.

When the SHRINK flag was finally implemented for the first backend
in 1.2.13 (commit aa9aa6a9), it was only implemented for the absolute
values and with the delta flag the volume is always extended,
regardless of the SHRINK flag.

Treat the SHRINK flag as a minus sign when used together with DELTA,
to allow shrinking volumes as was documented in the API since 0.9.10.

https://bugzilla.redhat.com/show_bug.cgi?id=1220213

src/storage/storage_driver.c

@@ -2320,7 +2320,10 @@ storageVolResize(virStorageVolPtr obj,
     }
 
     if (flags & VIR_STORAGE_VOL_RESIZE_DELTA) {
-        abs_capacity = vol->target.capacity + capacity;
+        if (flags & VIR_STORAGE_VOL_RESIZE_SHRINK)
+            abs_capacity = vol->target.capacity - MIN(capacity, vol->target.capacity);
+        else
+            abs_capacity = vol->target.capacity + capacity;
         flags &= ~VIR_STORAGE_VOL_RESIZE_DELTA;
     } else {
         abs_capacity = capacity;