bdrv_is_read_only() only checks if the node is configured to be
read-only eventually, but even if it returns false, writing to the node
may not be permitted at the moment (because it's inactive).

bdrv_is_writable() checks that the node can be written to right now, and
this is what the snapshot operations really need.

Change bdrv_can_snapshot() to use bdrv_is_writable() to fix crashes like
the following:

$ ./qemu-system-x86_64 -hda /tmp/test.qcow2 -loadvm foo -incoming defer
qemu-system-x86_64: ../block/io.c:1990: int bdrv_co_write_req_prepare(BdrvChild *, int64_t, int64_t, BdrvTrackedRequest *, int): Assertion `!(bs->open_flags & BDRV_O_INACTIVE)' failed.

The resulting error message after this patch isn't perfect yet, but at
least it doesn't crash any more:

$ ./qemu-system-x86_64 -hda /tmp/test.qcow2 -loadvm foo -incoming defer
qemu-system-x86_64: Device 'ide0-hd0' is writable but does not support snapshots

Signed-off-by: Kevin Wolf <kwolf@redhat.com>
---
 block/snapshot.c | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/block/snapshot.c b/block/snapshot.c
index ec8cf4810b..c4d40e80dd 100644
--- a/block/snapshot.c
+++ b/block/snapshot.c
@@ -196,8 +196,10 @@ bdrv_snapshot_fallback(BlockDriverState *bs)
 int bdrv_can_snapshot(BlockDriverState *bs)
 {
     BlockDriver *drv = bs->drv;
+
     GLOBAL_STATE_CODE();
-    if (!drv || !bdrv_is_inserted(bs) || bdrv_is_read_only(bs)) {
+
+    if (!drv || !bdrv_is_inserted(bs) || !bdrv_is_writable(bs)) {
         return 0;
     }
 
-- 
2.43.0