make vmtest fails on arm64: qemu-system-aarch64 has no default for -machine

`make vmtest` fails on arm64 (aarch64):

$ make vmtest
[...]
nosetests3 -vv --nologcapture tests/vmtests
nose.config: INFO: Ignoring files matching ['^\\.', '^_', '^setup\\.py$']
2019-04-24 10:51:29,447 - vmtests - INFO - Logfile: /tmp/vmtest-2019-04-24T105129.445408.log . Working dir: /tmp/vmtest-2019-04-24T105129.445408
nose.selector: INFO: /home/ubuntu/paride/curtin/tests/vmtests/report_webhook_logger.py is executable; skipped
2019-04-24 10:51:31,592 - BionicTestAptConfigCMDCMD - INFO - Logfile: /tmp/vmtest-2019-04-24T105129.445408.log . Working dir: /tmp/vmtest-2019-04-24T105129.445408
2019-04-24 10:51:31,599 - BionicTestAptConfigCMDCMD - INFO - Starting setup for testclass: BionicTestAptConfigCMDCMD (ubuntu/bionic -> ubuntu/bionic)
2019-04-24 10:51:31,821 - BionicTestAptConfigCMDCMD - INFO - Using tempdir: /tmp/vmtest-2019-04-24T105129.445408/BionicTestAptConfigCMDCMD
2019-04-24 10:51:31,837 - BionicTestAptConfigCMDCMD - INFO - Publishing ephemeral image as --publish=/srv/images/bionic/arm64/20190419/squashfs:root/squashfs
2019-04-24 10:51:31,838 - BionicTestAptConfigCMDCMD - INFO - Curtin install source URI: cp:///media/root-ro
2019-04-24 10:51:31,862 - BionicTestAptConfigCMDCMD - INFO - Loading testcase config file: examples/tests/apt_config_command.yaml
2019-04-24 10:51:31,887 - BionicTestAptConfigCMDCMD - INFO - disk_serials: []
2019-04-24 10:51:31,887 - BionicTestAptConfigCMDCMD - INFO - nvme_serials: []
2019-04-24 10:51:31,887 - BionicTestAptConfigCMDCMD - INFO - nvme disks: []
2019-04-24 10:51:31,921 - BionicTestAptConfigCMDCMD - INFO - Using root_disk: []
2019-04-24 10:51:31,922 - BionicTestAptConfigCMDCMD - INFO - Running curtin installer: /tmp/vmtest-2019-04-24T105129.445408/BionicTestAptConfigCMDCMD/logs/install-serial.log
2019-04-24 10:51:33,918 - BionicTestAptConfigCMDCMD - INFO - BionicTestAptConfigCMDCMD[install]: boot took 1.99 seconds. returned None
2019-04-24 10:51:33,926 - BionicTestAptConfigCMDCMD - WARNING - Boot for install did not produce a console log.
ERROR

The happens when tools/xkvm is called, failing with:

  bad device model virtio-blk?

This happens because tools/xkvm calls qemu-kvm like this:

  $KVM -device "$model,?"

expanded:

  qemu-system-aarch64 -device "virtio-blk,?"

which fails with:

  qemu-system-aarch64: No machine specified, and there is no default
  Use -machine help to list supported machines

so it seems that arm64 has to be handled in a special way, as there is no default for `-machine`.