s390/getty-generator: initialize essential system terminals/consoles

Ensure to start getty programs on all essential system consoles on Linux on
System z.  Add these essential devices to the list of virtualization_consoles
to always generate getty configurations.

For the sake of completion, the list of essential consoles is:

  /dev/sclp_line0 - Operating system messages applet (LPAR)
  /dev/ttysclp0 - Integrated ASCII console applet (z/VM and LPAR)
  /dev/ttyS0 - Already handled by systemd (3215 console on z/VM)
  /dev/hvc0  - Already handled by systemd (IUCV HVC terminal on z/VM)

Depending on the environment, z/VM or LPAR, only a subset of these terminals
are available.

See also RH BZ 860158[1] "Cannot login via Operating System Console into RHEL7
instance installed on a LPAR".  This bugzilla actually blocks the installation
of Linux on System z instances in LPAR mode.

[1] https://bugzilla.redhat.com/show_bug.cgi?id=860158