LXC: introduce virLXCControllerSetupUserns and lxcContainerSetID

LXC: introduce virLXCControllerSetupUserns and lxcContainerSetID

This patch introduces new helper function
virLXCControllerSetupUserns, in this function,
we set the files uid_map and gid_map of the init
task of container.

lxcContainerSetID is used for creating cred for
tasks running in container. Since after setuid/setgid,
we may be a new user. This patch calls lxcContainerSetUserns
at first to make sure the new created files belong to
right user.

Signed-off-by: Gao feng <gaofeng@cn.fujitsu.com>