fix for issue 1527 support for cygwin guest for windows

Hey,  I posted about this on the forums and would like to contribute this change for consideration on a future release of vagrant so out of the box cygwin works on windows for windows guest.

Please see this post for cygwin setup:

https://groups.google.com/forum/?fromgroups=#!topic/vagrant-up/TSpQFb2ixB0

Please view this post on reference for issues that were encountered when using cygwin guest.

https://groups.google.com/forum/?fromgroups=#!topic/vagrant-up/JxpfFD4KI8E

Thanks!