OK, it seems what happens is that xbd_diskstart() routine gets a
request for starting I/O bigger than it can handle. The NetBSD 9.0
xbd(4) has a limit of 44 KB maximum I/O, and XEN3_DOMU has specific
override for MAXPHYS to 32 KB for this reason.
It seems the ZFS module does not take this into account, and is
compiled with default 64 KB MAXPHYS.
On -current xbd(4) was changed to remove this limitation, and works
with default 64KB MAXPHYS. One option is then to just upgrade the
kernel to -current. You can leave userland on 9.0. If that is not
possible for you, you can build your own custom ZFS module with an
override of MAXPHYS=32768, using still netbsd-9 sources.