1) additional qemu tests for ami, vmdk, vhd, and openstack image types
2) new type of systemd-nspawn tests for tar, ext4, and parititioned disk
types
the systemd-nspawn tests use loopback network interface directly from
the host so it is necessary to tweak the settings of its SSH server.
This is done in a "script" stage using simple "sed" command.
