support for folder sync via rsync on windows.

rsync interprets paths with colons as remote locations.
This leads to *"The source and destination cannot both be remote."* erros when using local paths like `c:\vagrant` for the hostpath.
This commits converts the hostpath to /cygdrive/... notation on windows in general.
I confirmed that this works with the cwRsync Installer (https://www.itefix.no/i2/cwrsync).