block: Fix snapshot=on with aio=native

snapshot=on creates a temporary overlay that is always opened with
cache=unsafe (the cache mode specified by the user is only for the
actual image file and its children). This means that we must not inherit
the BDRV_O_NATIVE_AIO flag for the temporary overlay because trying to
use Linux AIO with cache=unsafe results in an error.

Reproducer without this patch:

$ x86_64-softmmu/qemu-system-x86_64 -drive file=/tmp/test.qcow2,cache=none,aio=native,snapshot=on
qemu-system-x86_64: -drive file=/tmp/test.qcow2,cache=none,aio=native,snapshot=on: aio=native was
specified, but it requires cache.direct=on, which was not specified.

Signed-off-by: Kevin Wolf <kwolf@redhat.com>

block.c

@@ -684,6 +684,10 @@ static void bdrv_temp_snapshot_options(int *child_flags, QDict *child_options,
     /* For temporary files, unconditional cache=unsafe is fine */
     qdict_set_default_str(child_options, BDRV_OPT_CACHE_DIRECT, "off");
     qdict_set_default_str(child_options, BDRV_OPT_CACHE_NO_FLUSH, "on");
+
+    /* aio=native doesn't work for cache.direct=off, so disable it for the
+     * temporary snapshot */
+    *child_flags &= ~BDRV_O_NATIVE_AIO;
 }
 
 /*