api: Add ExtendVirtualDisk method on VirtualDiskManager

[spacegospod]

Description

Adding a binding for extendVirtualDisk on VirtualDiskManager.

https://github.com/hashicorp/terraform-provider-vsphere/issues/851

Type of change

Please mark options that are relevant:

• [ ] Bug fix (non-breaking change which fixes an issue)
• [X] 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?

Extended the tests for virtual_disk_manager.go with new scenarios that cover 1.[POSITIVE] Extending a disk 2.[NEGATIVE] Attempting to shrink a disk 3.[NEGATIVE] Attempting to extend a disk that does not exist

The simulation for virtual disks did not include metadata for the disk size. I've modified it so that it writes the size value (in KB) in the file itself.

Manually verified the same scenarios on a real VMDK.

Checklist:

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

[spacegospod]

Thanks @spacegospod , lgtm. Although, VirtualDiskManager has been deprecated for quite some time. Has TF considered switching to First Class Disks (aka VStorageObjectManager) ? That'd be ExtendDisk in this case.

Hey @dougm I will try switching the TF provider to VStorageObjectManager. Lets still have this merged, just as a back up