For BlockBackends that are skipped in query-blockstats, we would leak
info since commit 567dcb31. Allocate info only later to avoid the memory
leak.

Fixes: CID 1394727
Cc: qemu-stable@nongnu.org
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Reviewed-by: Alberto Garcia <berto@igalia.com>