replication: Avoid blk_make_empty() on read-only child

replication: Avoid blk_make_empty() on read-only child

This is just a bandaid to keep tests/test-replication working after
bdrv_make_empty() starts to assert that we're not trying to call it on a
read-only child.

For the real solution in the future, replication should not steal the
BdrvChild from its backing file (this is never correct to do!), but
instead have its own child node references, with the appropriate
permissions.

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