Deactivate disks: allow skipping hypervisor checks

In some cases (e.g. the hypervisor not running at all), we might want
to force disk deactivation, skipping the hypervisor checks. I believe
this is not a good thing to do all the time, so this patch adds the
force option to allow manual selection of this operation mode.

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