Fix cgroups when all are mounted on /sys/fs/cgroup

Fix cgroups when all are mounted on /sys/fs/cgroup

Some users in Ubuntu/Debian seem to have a setup where all the
cgroup controllers are mounted on /sys/fs/cgroup rather than
any /sys/fs/cgroup/<controller> name. In the loop which detects
which controllers are present for a mount point we were modifying
'mnt_dir' field in the 'struct mntent' var, but not always restoring
the original value. This caused detection to break in the all-in-one
mount setup.

Fix that logic bug and add test case coverage for this mount
setup.

Signed-off-by: Daniel P. Berrange <berrange@redhat.com>