platforms: add secondary serial console on VirtualBox/VMware

The FCOS platform docs for VMware/VirtualBox include instructions on
connecting to the serial console to get a console log, and some users
make use of this functionality:

    coreos/fedora-coreos-tracker#567 (comment)

Re-enable secondary serial consoles on those platforms, so the serial
console gets as much information as possible without interfering with the
graphical console.

platforms.yaml

@@ -143,3 +143,23 @@ x86_64:
     kernel_arguments:
       - console=tty0
       - console=ttyS0,115200n8
+  virtualbox:
+    # Graphical console primary, serial console available for logging
+    # https://docs.fedoraproject.org/en-US/fedora-coreos/provisioning-virtualbox/#_troubleshooting_first_boot_problems
+    grub_commands:
+      - serial --speed=115200
+      - terminal_input serial console
+      - terminal_output serial console
+    kernel_arguments:
+      - console=ttyS0,115200n8
+      - console=tty0
+  vmware:
+    # Graphical console primary, serial console available for logging
+    # https://docs.fedoraproject.org/en-US/fedora-coreos/provisioning-vmware/#_troubleshooting_first_boot_problems
+    grub_commands:
+      - serial --speed=115200
+      - terminal_input serial console
+      - terminal_output serial console
+    kernel_arguments:
+      - console=ttyS0,115200n8
+      - console=tty0