Can we don't do unplug_vifs and plug_vifs when start vm?

[shews]

Hello all.
Now we use hard_reboot to do vm start.I think this is unnecessary, especially for the unplug_vifs and plug_vifs action, which will waste a lot of time and is even more unnecessary.

Can we optimize the code for vm start(the power_on function at virt/libvirt/driver.py), let it don't do unplug_vifs and plug_vifs, so that we can shorten the vm start time?

Is there any problem with this change? Can someone give some suggestions?