Add VZMemoryBalloonDevice Support

[walteh]

I ran into a case where I needed to adjust the memory partition of a VirtioTraditionalMemoryBalloonDevice, but found it not yet implemented here.

The ideal solution would be:

1. the ability to read the memoryBalloonDevices var on the VZVirtualMachine
2. the ability to read and write the targetVirtualMachineMemorySize var on the VZVirtioTraditionalMemoryBalloonDevice

Something along the lines of:

func (v *VirtualMachine) MemoryBalloonDevices() []VZMemoryBalloonDevice

func (b *VirtioTraditionalMemoryBalloonDevice) GetTargetVirtualMachineMemorySize() uint64
func (b *VirtioTraditionalMemoryBalloonDevice) SetTargetVirtualMachineMemorySize(uint64)

I tried hacking around it and nothing works as seamlessly as direct changes to this library.

As part of my hacking I made the changes in a fork and am using that for now and it's working as needed. At your request I am not submitting it as a PR and am instead opening this issue. However, I cleaned up the code and have created a draft reference pr in case it is at all helpful to anyone: walteh#1.

Thanks for all your work on this library, hope to sponsor you someday!

[cfergeau]

For what it’s worth, this API is the missing piece for memory balloon support in vfkit crc-org/vfkit#255

[Code-Hex]

@walteh Thanks for creating a issue!
Could you send me your PR? I can review it.

Thanks

[walteh]

@Code-Hex for sure! I just created it: #188

[Code-Hex]

@walteh Thanks! I will check later! (In today)