vconsole-setup: pass when uml is detected

User-Mode Linux does not do VT102 emulation, and

  $ systemd-vconsole-setup /dev/tty0

will always fail on it.  In order to prevent vconsole-setup.service from
always failing, write an exception for it in vconsole-setup.c.  Now,
vconsole-setup.service will pass on um Linux, and the logs will indicate
that /dev/tty0 is not a virtual console.

Cc: Kay Sievers <kay@vrfy.org>
Cc: Zbigniew Jędrzejewski-Szmek <zbyszek@in.waw.pl>
Cc: Lennart Poettering <lennart@poettering.net>
Signed-off-by: Ramkumar Ramachandra <artagnon@gmail.com>