libvirt: Bump MIN_{LIBVIRT,QEMU}_VERSION for "Stein"

In commit 28d337b[1], we advertized that the NEXT_MIN_LIBVIRT and
NEXT_MIN_QEMU_VERSION for "Stein" will be:

    libvirt 3.0.0 and QEMU 2.8.0

Now that we are in the development cycle for "Stein", bump the
MIN_{LIBVIRT,QEMU}_VERSION to the above agreed-upon versions.

As part of this clean-up:

  - Remove the _create_file_device() function and the calls to it; it is
    a "no-op" when 'virtlogd' is available.  As a result of this, in
    _create_consoles_s390x(), this patch entirely removes the "sclplm"
    serial console device — otherwise 'virtlogd', which is now always
    available, will create a duplicate 'pty' devices ("sclplm" and
    "sclp") pointing to the same log, which results in instance creation
    failure; we don't want that.

  - Remove the requirement for extra serial device (added in commit:
    1f65925: "libvirt: virtlogd: use virtlogd for char devices"), that
    "allows access to a Nova instance via `virsh console <guest>`" in
    _create_pty_device() — it is not required.  I also double-checked
    with libvirt and QEMU developer Daniel Berrangé, who said (slightly
    paraphrasing):

        "Nova should not allow `virsh console` to guests behind its back
        at all.  And especially it should not care about `virsh console`
        working with "tcp".  The point of using "tcp" consoles in Nova
        is that it provides tunneling via the Nova "serial console
        server".  You can only have 1 thing connected to a console at a
        time — so if the Nova serial console is present, `virsh console`
        can't be used anyway."

  - The unit test noise is largely mechanical: remove the superflous
    serial device, and lower the index of the devices by 1.

The following version constants (and corresponding tests), that are now
no longer required, will be removed in separate patches:

    MIN_LIBVIRT_PARALLELS_SET_ADMIN_PASSWD,
    MIN_LIBVIRT_POSTCOPY_VERSION, MIN_{LIBVIRT,QEMU}_LUKS_VERSION,
    MIN_QEMU_FILE_BACKED_VERSION, MIN_LIBVIRT_PERF_VERSION

[1] http://git.openstack.org/cgit/openstack/nova/commit/?id=28d337b --
    Pick next minimum libvirt / QEMU versions for "Stein"

Change-Id: I408baef12358a83921c4693b847a692f6c19e36f
Signed-off-by: Kashyap Chamarthy <kchamart@redhat.com>

  - The unit test noise is largely mechanical: remove the superflous
    serial device, and lower the index of the devices by 1.

The following version constants (and corresponding tests), that are now
no longer required, will be removed in separate patches:

    MIN_LIBVIRT_PARALLELS_SET_ADMIN_PASSWD,
    MIN_LIBVIRT_POSTCOPY_VERSION, MIN_{LIBVIRT,QEMU}_LUKS_VERSION,
    MIN_QEMU_FILE_BACKED_VERSION, MIN_LIBVIRT_PERF_VERSION

[1] http://git.openstack.org/cgit/openstack/nova/commit/?id=28d337b --
    Pick next minimum libvirt / QEMU versions for "Stein"

Change-Id: I408baef12358a83921c4693b847a692f6c19e36f
Signed-off-by: Kashyap Chamarthy <kchamart@redhat.com>

aborrero@buster-boot-arturo-01:~$ sudo systemctl status getty@ttyS1.service
● getty@ttyS1.service - Getty on ttyS1
   Loaded: loaded (/lib/systemd/system/getty@.service; enabled; vendor preset: enabled)
  Drop-In: /etc/systemd/system/getty@ttyS1.service.d
           └─puppet-override.conf
   Active: active (running) since Mon 2019-12-16 13:35:35 UTC; 6s ago
     Docs: man:agetty(8)
           man:systemd-getty-generator(8)
           http://0pointer.de/blog/projects/serial-console.html
 Main PID: 7219 (agetty)
    Tasks: 1 (limit: 4699)
   Memory: 260.0K
   CGroup: /system.slice/system-getty.slice/getty@ttyS1.service
           └─7219 /sbin/agetty --autologin root --noclear ttyS1 vt220

Dec 16 13:35:35 buster-boot-arturo-01 systemd[1]: Started Getty on ttyS1.
Dec 16 13:35:35 buster-boot-arturo-01 agetty[7219]: /dev/ttyS1: not a tty
aborrero@buster-boot-arturo-01:~$ file /dev/ttyS1
/dev/ttyS1: character special (4/65)