Have you written an nbdkit plugin that you're willing to share?
Email <libguestfs@redhat.com> to get it added to this list.

* XVA (XenServer Virtual Appliance)

This simple plugin written in Perl lets you read these files and
reexport them as NBD.  It does not require that the XVA is unpacked:
https://lists.gnu.org/archive/html/qemu-devel/2017-11/msg02971.html
https://lists.gnu.org/archive/html/qemu-devel/2017-11/plHKJ062x5Uo.pl

The whole thread is worth reading because several different approaches
to dealing with XVA files are presented:
https://lists.gnu.org/archive/html/qemu-devel/2017-11/threads.html#02839
