another invalid memory access, now xen: acpi-cnt

[Michael Tokarev]

See https://bugs.launchpad.net/qemu/+bug/1886318 , in particular
the #13 in there (the patch) - I applied this to qemu which crashes
after 
revert-memory-accept-mismatching-sizes-in-memory_region_access_valid-CVE-2020-13754.patch
when run as the device model for Xen HVM domU.  Here's the output:

invalid size: acpi-cnt addr 0 size: 1
invalid size: acpi-cnt addr 0 size: 1

after this it just dies.

This is another incarnation of LP#1886318 .
What can we do here?

Thanks,

/mjt