vcsim: Fix RefreshDatastore to return a valid response

[dekp]

Description

Fixed the implementation of RefreshDatastore in the VC simulator to set the response field when there is no fault (i.e. happy path) in order to avoid serialization problems.

Closes: #3198

Type of change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to
  not work as expected)
• This change requires a documentation update
• Build related change

How Has This Been Tested?

• Modified the existing unit test for RefreshDatastore to validate the response field.
• Developer testing via standalone govmomi-based client against vcsim.

Checklist:

• My code follows the CONTRIBUTION guidelines of this project
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• Any dependent changes have been merged

[dougm]

Thanks @dekp