core: use process.alive? to avoid exceptions

CHANGELOG.md

@@ -19,6 +19,7 @@ BUG FIXES:
   - core: Setting an ID on a provisioner now works. [GH-3424]
   - core: All synced folder paths containing symlinks are fully
       expanded before sharing. [GH-3444]
+  - core: Windows no longer sees "process not started" errors rarely.
   - commands/box/repackage: Works again. [GH-3372]
   - commands/box/update: Update should check for updates from latest
       version. [GH-3452]

lib/vagrant/util/subprocess.rb

@@ -204,7 +204,7 @@ module Vagrant
         # Return an exit status container
         return Result.new(process.exit_code, io_data[:stdout], io_data[:stderr])
       ensure
-        if process && !process.exited?
+        if process && process.alive?
           # Make sure no matter what happens, the process exits
           process.stop(2)
         end