Currently when handling 'container_path' elements in container mounts we simply call
filepath.Clean on those paths. However, filepath.Clean adds an extra '.' if the path is a
simple drive letter ('E:' or 'Z:' etc.). These type of paths cause failures (with incorrect
parameter error) when creating containers via hcsshim. This commit checks for such paths
and doesn't call filepath.Clean on them.
It also adds a new check to error out if the destination path is a C drive and moves the
dst path checks out of the named pipe condition.
Signed-off-by: Amit Barve <ambarve@microsoft.com>