fmtarches: Use dom0arches, not hardcoded arch list

fmtarches: Use dom0arches, not hardcoded arch list

This will make us reallocate fmt tests when the arch list changes.
It's not ideal because it means tests jumping about across arches and
might let regressions go through but it's better than just dropping
them, and doing a better approach is complex.

This changes some jobs and adds others, roughly:

  +               test-amd64-i386-libvirt-fraw
  -               test-amd64-i386-xl-raw
  +               test-amd64-i386-xl-vhd
  +               test-arm64-arm64-libvirt-raw
  +               test-arm64-arm64-xl-vhd
  +               test-armhf-armhf-libvirt-qcow2

(as reported by standalone-generate-dump-flight-runvars)

Signed-off-by: Ian Jackson <iwj@xenproject.org>