fix invalid esx5 path separator in windows

before, this code had joining path elements by filepath module. filepath module generate path string with backslash-joined in Windows. but ESX require path string with slash-joined. it means that this code generate illegal path string in windows. illegal path string raised "Error creating disk".

this patch fixes path separator from backslash to slash in windows. from this, creating disk would succeed without error.
Merge pull request #1316 from yuuzi41/fix-esx5-path-separator

fix invalid esx5 path separator in windows