vconsole-setup: setup negative conditional on uml

User-Mode Linux does not do VT102 emulation, and

  $ vconsole-setup /dev/tty0

will always fail on it.  In order to prevent vconsole-setup.service from
always failing, disable it when uml is detected.

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