Hop to the correct label when reboot is requested.

In #674 we changed the label which initiated reboot from `reboot` to
`prep_reboot`. We do reboot in 2 cases:

1. When setting up host
2. On VmHost.incr_reboot

We had updated the label to be hopped at for case #1, but we had missed
updating the label for case #2. This PR fixes that.

prog/vm/host_nexus.rb

@@ -182,7 +182,7 @@ def before_run
 
   label def wait
     when_reboot_set? do
-      hop_reboot
+      hop_prep_reboot
     end
 
     nap 30

spec/prog/vm/host_nexus_spec.rb

@@ -218,9 +218,9 @@
       expect { nx.wait }.to nap(30)
     end
 
-    it "hops to reboot when needed" do
+    it "hops to prep_reboot when needed" do
       expect(nx).to receive(:when_reboot_set?).and_yield
-      expect { nx.wait }.to hop("reboot")
+      expect { nx.wait }.to hop("prep_reboot")
     end
   end