detect-virt: detect User-Mode Linux

In a User-Mode Linux session:

  $ systemd-detect-virt
  none

Although it is possible to reliably detect virtualization:

  $ cat /proc/cpuinfo
  processor       : 0
  vendor_id       : User Mode Linux
  model name      : UML
  mode            : skas
  host            : Linux kytes 3.11.0-rc1-00009-ge5fd680 (...)
  bogomips        : 7007.43

So, grep for the string "\nvendor_id\t: User Mode Linux\n" in
/proc/cpuinfo, and say "uml" when asked.

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