Happened once so far.

Guest: openwrt-18.06.0-rc1-x86-64-combined-ext4.img under paravirtualisation,
booted with PvGrub2 (instructions:
https://wiki.xen.org/wiki/PvGrub2#Loading_grub.cfg_from_any_partition)

On closing the VM, qemu crashed:

#0  0x00007f3eb4bfc941 in __strlen_avx2 () from /lib64/libc.so.6
#1  0x000055e68495140f in xenstore_update_fe (watch=0x55e6870fef90
"/local/domain/7/device/vfb/0", xendev=0x55e6870f5660) at
hw/xen/xen_backend.c:507
#2  0x000055e68495217d in xenstore_update (unused=<optimized out>) at
hw/xen/xen_pvdev.c:170
#3  0x000055e684a98ce2 in aio_dispatch_handlers (ctx=ctx@entry=0x55e6870da8c0)
at util/aio-posix.c:406
#4  0x000055e684a99578 in aio_dispatch (ctx=0x55e6870da8c0) at
util/aio-posix.c:437
#5  0x000055e684a961ee in aio_ctx_dispatch (source=<optimized out>,
callback=<optimized out>, user_data=<optimized out>) at util/async.c:261
#6  0x00007f3eba1430be in g_main_context_dispatch () from
/usr/lib64/libglib-2.0.so.0
#7  0x000055e684a98878 in glib_pollfds_poll () at util/main-loop.c:215
#8  os_host_main_loop_wait (timeout=<optimized out>) at util/main-loop.c:263
#9  main_loop_wait (nonblocking=<optimized out>) at util/main-loop.c:522
#10 0x000055e6846808be in main_loop () at vl.c:1944
#11 main (argc=<optimized out>, argv=<optimized out>, envp=<optimized out>) at
vl.c:4746
(gdb) up
#1  0x000055e68495140f in xenstore_update_fe (watch=0x55e6870fef90
"/local/domain/7/device/vfb/0", xendev=0x55e6870f5660) at
hw/xen/xen_backend.c:507
507         len = strlen(xendev->fe);
(gdb) p xendev->fe
$1 = 0x0