Check early (but after grabbing a startup_lock) if VM isn't just
removed. This could happen if someone grabs its reference from other
places (netvm of something else?) or just before removing it.
This commit makes the simple removal from the collection (done as the
first step in admin.vm.Remove implementation) efficient way to block
further VM startups, without introducing extra properties.
For this to be effective, removing from the collection, needs to happen
with the startup_lock held. Modify admin.vm.Remove accordingly.
732231efb0