configure: Add function to enable serial console

The `enable_console_login` function modifies `/etc/inittab` to enable
logins on a serial device. For virtio serial devices, a getty is
configured to launch on `hvc0`; for other devices, it is configured for
`ttyS0` (the default).

When using the `configure.configure` function to automatically configure
a virtual machine, `enable_console_login` is called if a console device
is specified in the VM definition.

src/mkvm/configure.py

@@ -1,6 +1,7 @@
 import glob
 import logging
 import os
+import re
 import subprocess
 
 
@@ -53,6 +54,27 @@ def set_hostname(mountpoint, hostname):
                 f.write(line)
 
 
+def enable_console_login(mountpoint, driver):
+    path = os.path.join(mountpoint, 'etc/inittab')
+    with open(path) as f:
+        lines = f.readlines()
+    regex = re.compile(r'^#?s0')
+    entry = 's0:{runlevels}:{action}:{process}'
+    with open(path, 'w') as f:
+        for line in lines:
+            if regex.match(line):
+                runlevels, action, process = line.split(':', 3)[1:]
+                if driver == 'virtio':
+                    process = process.replace('ttyS0', 'hvc0')
+                f.write(entry.format(
+                    runlevels=runlevels,
+                    action=action,
+                    process=process,
+                ))
+            else:
+                f.write(line)
+
+
 def run_custom_script(mountpoint, script):
     cmd = ['chroot', mountpoint, '/bin/sh']
     env = CUSTOM_SCRIPT_ENV.copy()
@@ -74,6 +96,8 @@ def configure(vm):
         set_timezone(vm.mountpoint, vm.timezone)
     if vm.fqdn:
         set_hostname(vm.mountpoint, vm.fqdn)
+    if vm.console:
+        enable_console_login(vm.mountpoint, vm.console)
     if vm.customize:
         run_custom_script(vm.mountpoint, vm.customize)