Update image validation code to accept VHDX ImageFormat.
Update error message when image file extension is invalid to better
distinguish where in the code it is coming from and to better explain
what the problem is.
Add 'vhdx' as an option for RegisterTemplate's image types.
Applies to Hyper-V only.
Merge branch 'master' of https://github.com/lafferty/cshv3
Time accuracy on Windows systems is less than 15.625ms in cases, thus test's
pause needs to be increased.