hypervisor support matrix: add feature "serial console"

The "serial console" feature was introduced during the Juno cycle
and is an alternative to remote access via VNC, RDP, SPICE for
platforms which don't support these.
This change adds a new feature row for that to the hypervisor support
matrix table. Because I'm not able to make a statement for all
hypervisor targets I use the "unknown" state for them.

Change-Id: I23381e30b6ddb823a50f7755179eb02bc18222e8

doc/source/support-matrix.ini

@@ -706,6 +706,35 @@ driver-impl-ironic=missing
 driver-impl-libvirt-parallels-vm=missing
 driver-impl-libvirt-parallels-ct=missing
 
+[console.serial.interactive]
+title=Remote interactive serial console
+status=choice(console)
+notes=This allows the administrator to interact with the serial
+  console of the guest OS. This provides a way to see boot
+  up messages and login to the instance when networking configuration
+  has failed, thus preventing a network based login. Not all guest
+  operating systems will be able to emit boot information on a serial
+  console, others may only support graphical consoles. Therefore support
+  for this operation is not mandatory, however, a driver is required to
+  support at least one of the listed console access operations.
+  This feature was introduced in the Juno release with blueprint
+  https://blueprints.launchpad.net/nova/+spec/serial-ports
+cli=nova get-serial-console <server>
+driver-impl-xenserver=missing
+driver-impl-libvirt-kvm-x86=complete
+driver-impl-libvirt-kvm-ppc64=unknown
+driver-impl-libvirt-kvm-s390x=complete
+driver-impl-libvirt-qemu-x86=unknown
+driver-impl-libvirt-lxc=unknown
+driver-impl-libvirt-xen=unknown
+driver-impl-vmware=missing
+driver-impl-hyperv=missing
+driver-notes-hyperv=Will be complete when this review is merged:
+  https://review.openstack.org/#/c/145004/
+driver-impl-ironic=unknown
+driver-impl-libvirt-parallels-vm=unknown
+driver-impl-libvirt-parallels-ct=unknown
+
 [console.spice]
 title=Remote desktop over SPICE
 status=choice(console)