Lots of code expects the VM to be Halted after receiving one of these
events, but it could also be Dying or Crashed. Get rid of the Dying case
at least, by waiting until the VM has transitioned out of it.
Fixes e.g. the following DispVM cleanup bug:
$ qvm-create -C DispVM --prop auto_cleanup=True -l red dispvm
$ qvm-start dispvm
$ qvm-shutdown --wait dispvm # this won't remove dispvm
$ qvm-start dispvm
$ qvm-kill dispvm # but this will
f96fd70f76