console logging does not work for OL instances on xen compute

[James McCarthy]

Testing with pike on an OVM 3.4.4 xen compute node, Oracle Linux instances do not have working console logging in horizon. (on kvm compute, they are fine)

Note: The OL 7.3 image used to create these instances is marked with --property vm_mode=hvm

On kvm the relevant configuration is similar to this:
    <serial type='file'>
      <source path='/var/lib/nova/instances/210817e9-9579-425d-a1f6-366c64826bc8/console.log'/>
      <target port='0'/>
      <alias name='serial0'/>
    </serial>
    <serial type='pty'>
      <source path='/dev/pts/11'/>
      <target port='1'/>
      <alias name='serial1'/>
    </serial>
    <console type='file'>
      <source path='/var/lib/nova/instances/210817e9-9579-425d-a1f6-366c64826bc8/console.log'/>
      <target type='serial' port='0'/>
      <alias name='serial0'/>
    </console>

By making experimental changes (adding in "xen") to the create_console method in https://github.com/openstack/nova/blob/master/nova/virt/libvirt/driver.py#L4569

    def _create_consoles(self, virt_type, guest_cfg, instance, flavor,
                         image_meta):
        # NOTE(markus_z): Beware! Below are so many conditionals that it is
        # easy to lose track. Use this chart to figure out your case:
        #
        # case | is serial | has | is qemu | resulting
        # | enabled? | virtlogd? | or kvm? | devices
        # --------------------------------------------------
        # 1 | no | no | no | pty*
        # 2 | no | no | yes | file + pty
        # 3 | no | yes | no | see case 1
        # 4 | no | yes | yes | pty with logd
        # 5 | yes | no | no | see case 1
        # 6 | yes | no | yes | tcp + pty
        # 7 | yes | yes | no | see case 1
        # 8 | yes | yes | yes | tcp with logd
        # * exception: virt_type "parallels" doesn't create a device
        if virt_type == 'parallels':
            pass
        elif virt_type not in ("qemu", "kvm", "xen"):
            log_path = self._get_console_log_path(instance)
            self._create_pty_device(guest_cfg,
                                    vconfig.LibvirtConfigGuestConsole,
                                    log_path=log_path)
        elif (virt_type in ("qemu", "kvm") and
                  self._is_s390x_guest(image_meta)):
            self._create_consoles_s390x(guest_cfg, instance,
                                        flavor, image_meta)
        elif virt_type in ("qemu", "kvm", "xen"):
            self._create_consoles_qemu_kvm(guest_cfg, instance,
                                        flavor, image_meta)

This allows for the creation of similar configuration on xen, which then looks to allow the console logging to work correctly.

The instance also had a modified /etc/default/grub, like so:
...
GRUB_TERMINAL="serial console"
GRUB_SERIAL_COMMAND="serial --speed=115200 --unit=0 --word=8 --parity=no --stop=1"
GRUB_CMDLINE_LINUX="console=tty0 console=ttyS0,115200n8"

I do not know if this works correctly in all cases, a more complete test matrix may be needed here.