vcluster: Don't virtualize /etc/hosts path

/etc/hosts is a bit special as it's a system-wide file and the virtual
cluster/node root doesn't apply. The modification of /etc/hosts should
be disabled in virtual clusters. If it isn't, however, the vcluster
functions would raise an exception complaining about a path outside of
the virtual node root. This patch adds a whitelist to exclude
/etc/hosts.

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