vcsim: support host disconnect/reconnect

[rcw5]

Is your feature request related to a problem? Please describe.

vcsim doesn't support disconnecting or reconnecting hosts, and so commands to perform these operations fail with a 'not implemented' error:

govc: ServerFaultCode: HostSystem:host-34 does not implement: ReconnectHost_Task
govc: ServerFaultCode: HostSystem:host-34 does not implement: DisconnectHost_Task

Describe the solution you'd like

vcsim should support these operations in the same way it supported entering/exiting maintenance mode

[github-actions]

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