Alright, with a d06 aarch64 machine I was able to reproduce it after 8 attempts.I'll debug it today and provide feedback on my findings. 

(gdb) bt full
#0  0x0000ffffb0b2181c in __GI_ppoll (fds=0xaaaace5ab770, nfds=4, timeout=<optimized out>, timeout@entry=0x0,
    sigmask=sigmask@entry=0x0) at ../sysdeps/unix/sysv/linux/ppoll.c:39
        _x3tmp = 0
        _x0tmp = 187650583213936
        _x0 = 187650583213936
        _x3 = 0
        _x4tmp = 8
        _x1tmp = 4
        _x1 = 4
        _x4 = 8
        _x2tmp = <optimized out>
        _x2 = 0
        _x8 = 73
        _sys_result = <optimized out>
        _sys_result = <optimized out>
        sc_cancel_oldtype = 0
        sc_ret = <optimized out>
        tval = {tv_sec = 0, tv_nsec = 187650583137792}
#1  0x0000aaaacd2a773c in ppoll (__ss=0x0, __timeout=0x0, __nfds=<optimized out>, __fds=<optimized out>)
    at /usr/include/aarch64-linux-gnu/bits/poll2.h:77
No locals.
#2  qemu_poll_ns (fds=<optimized out>, nfds=<optimized out>, timeout=timeout@entry=-1) at ./util/qemu-timer.c:322
No locals.
#3  0x0000aaaacd2a8764 in os_host_main_loop_wait (timeout=-1) at ./util/main-loop.c:233
        context = 0xaaaace599d90
        ret = <optimized out>
        context = <optimized out>
        ret = <optimized out>
#4  main_loop_wait (nonblocking=<optimized out>) at ./util/main-loop.c:497
        ret = <optimized out>
        timeout = 4294967295
        timeout_ns = <optimized out>
#5  0x0000aaaacd1df454 in convert_do_copy (s=0xfffff9b2b1d8) at ./qemu-img.c:1981
        ret = <optimized out>
        i = <optimized out>
        n = <optimized out>
        sector_num = <optimized out>
        ret = <optimized out>
        i = <optimized out>
        n = <optimized out>
        sector_num = <optimized out>
#6  img_convert (argc=<optimized out>, argv=<optimized out>) at ./qemu-img.c:2457
        c = <optimized out>
        bs_i = <optimized out>
        flags = 16898
        src_flags = 0
        fmt = 0xfffff9b2bad1 "qcow2"
        out_fmt = <optimized out>
        cache = 0xaaaacd2cb1c8 "unsafe"
        src_cache = 0xaaaacd2ca9c0 "writeback"
        out_baseimg = <optimized out>
        out_filename = <optimized out>
        out_baseimg_param = <optimized out>
        snapshot_name = 0x0
        drv = <optimized out>
        proto_drv = <optimized out>
        bdi = {cluster_size = 65536, vm_state_offset = 32212254720, is_dirty = false, unallocated_blocks_are_zero = true,
          needs_compressed_writes = false}
        out_bs = <optimized out>
        opts = 0xaaaace5ab390
        sn_opts = 0x0
        create_opts = 0xaaaace5ab0c0
        open_opts = <optimized out>
        options = 0x0
        local_err = 0x0
        writethrough = false
        src_writethrough = false
        quiet = <optimized out>
        image_opts = false
        skip_create = false
        progress = <optimized out>
        tgt_image_opts = false
        ret = <optimized out>
        force_share = false
        explict_min_sparse = false
        s = {src = 0xaaaace577240, src_sectors = 0xaaaace577300, src_num = 1, total_sectors = 62914560,allocated_sectors = 9572096, allocated_done = 6541440, sector_num = 8863744, wr_offs = 8859776, status = BLK_DATA, sector_next_status = 8863744, target = 0xaaaace5bd2a0, has_zero_init = true,compressed = false, unallocated_blocks_are_zero = true, target_has_backing = false, target_backing_sectors = -1, wr_in_order = true, copy_range = false, min_sparse = 8, alignment = 8,cluster_sectors = 128, buf_sectors = 4096, num_coroutines = 8, running_coroutines = 8, co = {0xaaaace5ceda0,0xaaaace5cef50, 0xaaaace5cf100, 0xaaaace5cf2b0, 0xaaaace5cf460, 0xaaaace5cf610, 0xaaaace5cf7c0,0xaaaace5cf970, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0}, wait_sector_num = {-1, 8859904, 8860928, 8863360,8861952, 8862976, 8862592, 8861440, 0, 0, 0, 0, 0, 0, 0, 0}, lock = {locked = 0, ctx = 0x0, from_push = {slh_first = 0x0}, to_pop = {slh_first = 0x0}, handoff = 0, sequence = 0, holder = 0x0}, ret = -115}
        __PRETTY_FUNCTION__ = "img_convert"
#7  0x0000aaaacd1d8400 in main (argc=7, argv=<optimized out>) at ./qemu-img.c:4976
        cmd = 0xaaaacd34ad78 <img_cmds+80>
        cmdname = <optimized out>
        local_error = 0x0
        trace_file = 0x0
        c = <optimized out>
        long_options = {{name = 0xaaaacd2cbbb0 "help", has_arg = 0, flag = 0x0, val = 104}, {
            name = 0xaaaacd2cbc78 "version", has_arg = 0, flag = 0x0, val = 86}, {name = 0xaaaacd2cbc80 "trace",
            has_arg = 1, flag = 0x0, val = 84}, {name = 0x0, has_arg = 0, flag = 0x0, val = 0}}