Comment 4 for bug 1848556

The stuck poll is at:
#0 0x00007fafb935ad26 in __GI_ppoll (fds=0x560dba615670, nfds=1, timeout=<optimized out>, timeout@entry=0x0, sigmask=sigmask@entry=0x0) at ../sysdeps/unix/sysv/linux/ppoll.c:39
#1 0x0000560db89550b9 in ppoll (__ss=0x0, __timeout=0x0, __nfds=<optimized out>, __fds=<optimized out>) at /usr/include/x86_64-linux-gnu/bits/poll2.h:77
#2 qemu_poll_ns (fds=<optimized out>, nfds=<optimized out>, timeout=<optimized out>) at ./util/qemu-timer.c:322
#3 0x0000560db89570eb in aio_poll (ctx=ctx@entry=0x560dba5e83b0, blocking=blocking@entry=true) at ./util/aio-posix.c:666
#4 0x0000560db888c21d in bdrv_check (bs=<optimized out>, res=<optimized out>, fix=<optimized out>) at ./block.c:4149
#5 0x0000560db887e6ab in collect_image_check (bs=0x560dba5ed680, check=0x560dba6143d0, filename=0x7ffe3d7c48d7 "http://localhost:80/bionic-server-cloudimg-amd64.img", fix=<optimized out>,
    fmt=<optimized out>) at ./qemu-img.c:615
#6 0x0000560db88825e1 in img_check (argc=<optimized out>, argv=<optimized out>) at ./qemu-img.c:774
#7 0x0000560db887bd2e in main (argc=2, argv=<optimized out>) at ./qemu-img.c:4987

And from strace we know that the FD is from
260 [pid 20469] 0.000067 eventfd2(0, EFD_CLOEXEC|EFD_NONBLOCK) = 8 <0.000041>