[BUG] vcsim: devices stay connected when vm is powered off

[Syuparn]

Describe the bug

When vm is powered off, its devices remain to be connected. They should be disconnected.

To Reproduce
Steps to reproduce the behavior:

1. run vcsim
2. connect a device on a powered on vm
3. power off the vm

$ vcsim &
$ govc vm.info DC0_H0_VM0 | grep Power
  Power state:  poweredOn

# connect ethernet-0
$ govc device.connect -vm DC0_H0_VM0 ethernet-0
$ govc device.info -vm DC0_H0_VM0 ethernet-0 | grep onnected
  Connected:        true
  Start connected:  true

# power off vm
$ govc vm.power -off DC0_H0_VM0
Powering off VirtualMachine:vm-54... OK

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

Device ethernet-0 is disconnected.

$ govc device.info -vm DC0_H0_VM0 ethernet-0 | grep onnected
  Connected:        false
  Start connected:  true

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

• vcsim v0.27.4

Screenshots/Debug Output
If applicable, add screenshots or debug output to help explain your problem.

Device ethernet-0 is still connected.

$ govc device.info -vm DC0_H0_VM0 ethernet-0 | grep onnected
  Connected:        true
  Start connected:  true

Additional context
Add any other context about the problem here.

• devices on suspended vm should also be disconnected

# real vCenter
$ govc device.info -vm myvm ethernet-1 | grep onnected
  Connected:        true
  Start connected:  true
$ govc vm.power -suspend myvm
Suspend VirtualMachine:vm-15... OK
$ govc device.info -vm myvm ethernet-1 | grep onnected
  Connected:        false
  Start connected:  true

• devices on powered off vm should not be connected

$ govc device.info -vm myvm ethernet-1 | grep onnected
  Connected:        false
  Start connected:  true
$ govc device.connect -vm myvm ethernet-1
$ govc device.info -vm myvm ethernet-1 | grep onnected
  Connected:        false
  Start connected:  true

• devices on powered off vm should be connected by vm power-on if Start connected is true

$ govc device.info -vm myvm ethernet-1 | grep onnected
  Connected:        false
  Start connected:  true
$ govc vm.power -on myvm
Powering on VirtualMachine:vm-15... OK
$ govc device.info -vm myvm ethernet-1 | grep onnected
  Connected:        true
  Start connected:  true