Implement upstream support for virtio-mem on s390x in QEMU, which includes support for the virtio-mem-ccw device.

Expected functionality that will be working:

• Memory hotplug/unplug
• Device hotplug/unplug
• System resets / reboots
• Migrate to/from file (including storage attributes under KVM)
• Basic live migration
• Basic postcopy live migration

Latest upstream proposal: [PATCH v2 00/14] s390x: virtio-mem support