In the past there was a cmdlet Refresh-LibraryShare that would allow you to refresh the library cache for an SCVMM server.
It no longer exists in the current VirtualMachineManager module.
I can see from this list: http://marcoshaw.blogspot.com/2011/03/virtual-machine-manager-changes-from.html when it was removed, with nothing to replace it.
Does anyone know how to perform a library refresh using the current SCVMM module? (or any other way?)