use qemu_free() instead of free()

There is a call to free() where qemu_free() should instead be used.
Signed-off-by: NBruce Rogers <brogers@novell.com>
Signed-off-by: NKevin Wolf <kwolf@redhat.com>

block.c

@@ -2075,7 +2075,7 @@ int bdrv_aio_multiwrite(BlockDriverState *bs, BlockRequest *reqs, int num_reqs)
     return 0;
 
 fail:
-    free(mcb);
+    qemu_free(mcb);
     return -1;
 }