Add KVM chroot feature

This patch adds a new boolean hypervisor parameter to the KVM hypervisor,
named 'use_chroot'.
If it's turned on for an instance, than KVM is started in "chroot mode":
Ganeti creates an empty directory for the instance and passes the path
of this dir to KVM via the -chroot flag.
KVM changes its root to this directory after starting up.

It also adds a "quarantine" feature for moving any unexpected files to
a separate directory for later analysis.

This has been backported from master,
commit 84c08e4e

Signed-off-by: Balazs Lecz <leczb@google.com>
Reviewed-by: Michael Hanselmann <hansmi@google.com>