getty-generator: run console shell for uml

In its default configuration, um Linux connects its /dev/console to the
host's fd:0,fd:1.  Other tty devices (/dev/tty*) are accessible on
various host devices that the user must connect to explicitly.  By
running getty on /dev/console by default,

  $ ./linux ubd0=arch-rootfs

will display a prompt immediately on the same terminal, along with the
boot messages.  This is much more user-friendly, than making the user
explicitly connect to the right device using

  $ screen /dev/pts/8

to get a login prompt.

Cc: Thomas Bächler <thomas@archlinux.org>
Cc: Lennart Poettering <lennart@poettering.net>
Signed-off-by: Ramkumar Ramachandra <artagnon@gmail.com>