qemu-system-s390x is only packaged on s390x, so only build it on s390x

I'll make it less special and behave like the others with the coming 2.10 upload.
Change is queued but waits for 2.10 to become stable enough for an upload.

Tested patch as it is committed in containers vs an unpatched version.
Fixes the case, thanks for your report Dannf!

This bug was fixed in the package qemu - 1:2.10~rc3+dfsg-0ubuntu1

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qemu (1:2.10~rc3+dfsg-0ubuntu1) artful; urgency=medium

  * Merge with Debian unstable (2.8) and Upstream 2.10-rci3; This fixes
    a set of bugs
    - [FFE] Qemu 2.10 in Artful (LP: #1699968)
    - CPU hot unplug fails after migrating a CPU hotplugged guest
      from source (LP: #1677552)
    - [Feature] KNL/KNM: Numa Distance on KVM(LP: #1647902)
    - New KVM 288 Pass Through (LP: #1672447)
    - aarch64: MSI is not supported by interrupt controller (LP: #1706630)
  * Remaining changes:
    - qemu-kvm to systemd unit
      - d/qemu-kvm-init: script for QEMU KVM preparation modules, ksm,
        hugepages and architecture specifics
      - d/qemu-kvm.service: systemd unit to call qemu-kvm-init
      - d/qemu-system-common.install: install systemd unit and helper script
      - d/qemu-system-common.maintscript: clean old sysv and upstart scripts
      - d/qemu-system-common.qemu-kvm.default: defaults for
        /etc/default/qemu-kvm
      - d/rules: install /etc/default/qemu-kvm
    - Enable nesting by default
      - set nested=1 module option on intel. (is default on amd)
      - re-load kvm_intel.ko if it was loaded without nested=1
      - d/p/ubuntu/expose-vmx_qemu64cpu.patch: expose nested kvm by default
        in qemu64 cpu type.
      - d/p/ubuntu/enable-svm-by-default.patch: Enable nested svm by default
        in qemu64 on amd
    - libvirt/qemu user/group support
      - qemu-system-common.postinst: remove acl placed by udev, and add udevadm
        trigger.
      - qemu-system-common.preinst: add kvm group if needed
    - Distribution specific machine type
      - d/p/ubuntu/define-ubuntu-machine-types.patch: define distro machine
        types to ease future live vm migration.
      - d/qemu-system-x86.NEWS Info on fixed machine type defintions
    - improved dependencies
      - Make qemu-system-common depend on qemu-block-extra
      - Make qemu-utils depend on qemu-block-extra
      - let qemu-utils recommend sharutils
    - s390x support
      - Create qemu-system-s390x package
      - Include s390-ccw.img firmware
      - Enable numa support for s390x
    - ppc64[le] support
      - d/qemu-system-ppc.links provide usr/bin/qemu-system-ppc64le symlink
      - Enable seccomp for ppc64el
      - bump libseccomp-dev dependency, 2.3 is the minimum for ppc64
    - arch aware kvm wrappers
    - disable missing x32 architecture
    - update VCS links
  * Added changes
      - d/rules: or32 is now named or1k (since 4a09d0bb)
      - d/qemu-system-common.docs: new paths since (ac06724a)
      - d/qemu-system-common.install: qmp-commands.txt removed, but replaced
        by qapi-schema.json which is already packaged (since 4d8bb958)
      - Updates in debian/patches to match qemu 2.10
        - d/p/02_kfreebsd.patch: utimensat is no more optional upstream
        - d/p/ubuntu/enable-svm-by-default.patch: target-i386 -> target/i386
        - d/p/ubuntu/expose-vmx_qemu64cpu.patch: target-i386 -> target/i386
        - d/p/ubuntu/define-ubuntu-machine-types.patch: new 2.10 ubuntu types
        - update VCS-git to match the Artful branch
      - s390x pack...