Merge branch 'stable-2.1' into devel-2.1

* stable-2.1:
  Release version 2.1.1
  Update NEWS file for the 2.1.1 release
  Validate the os-specific hypervisor parameters

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Michael Hanselmann <hansmi@google.com>