Support cygwin style file path for VBoxManage arguments

In cygwin world, file path style is like UNIX style. ("c:\foo\bar" -> "/cygdrive/c/foo/bar")
But normal windows executable cannot recognize this style, so before execute VBoxManage, convert from cygwin style path to windows style are needed.

- check RbConfig::CONFIG['host_os'] for cygwin world or not
- if in cygwin world, before execute VBoxManager, convert file path by cygpath tool