Add a class named RemoteFile in virttest-remote module.

A class to handle remote file.
Currently, we support three operations on remote file.
add: append lines in to file.
sub: replace string which match pattern to repl.
remove: delete lines that match pattern.

Signed-off-by: yangdongsheng <yangds.fnst@cn.fujitsu.com>
Add some unittest for RemoteFile.

Signed-off-by: yangdongsheng <yangds.fnst@cn.fujitsu.com>