[BUG] VirtualDeviceList.AssignController may generate positive device key

[Syuparn]

Describe the bug

VirtualDeviceList.AssignController should generate a negative random device key so that it will not collide with existing devices' keys. However, it sometimes generates positive key.

To Reproduce

1. run code below

// key has not been set to disk yet (disk.Key == 0)
disk := &types.VirtualDisk{
	VirtualDevice: types.VirtualDevice{},
}

l := object.VirtualDeviceList{disk}
c, _ := l.CreateIDEController()

// AssignContoller gives a new random key to device if it does not have a key yet
l.AssignController(disk, c.(types.BaseVirtualController))

fmt.Printf("disk device key: %d\n", disk.Key)

Expected behavior
A clear and concise description of what you expected to happen.

AssignController always gives negative keys to the device.

Affected version
Please provide details on the version used, e.g. release tag, commit, module version, etc.

• govmomi v0.28.0

Screenshots/Debug Output

AssignController sometimes gives a positive key to the device.

// key is positive
disk device key: 1697971134

Additional context
Add any other context about the problem here.

This is same issue as #2536, CreateEthernetCard sometimes generated a positive device key.

Generated key is expected to be negative (#2156), but type convertion from uint32 to int32 may change the key's sign.

[Syuparn]

Logic in #2569 can be used for this too. I will make a PR.