Attempt to read guest additions version from showvminfo [GH-1575]

CHANGELOG.md

@@ -12,6 +12,8 @@ IMPROVEMENTS:
   - Better locking around handling the `box_url` parameter for
     parallel providers.
   - Solaris guest is now properly detected on SmartOS, OmniOS, etc. [GH-1639]
+  - Guest addition version detection is more robust, attempting other
+    routes to get the version, and also retrying a few times. [GH-1575]
 
 BUG FIXES:
 

plugins/providers/virtualbox/driver/version_4_2.rb

@@ -264,6 +264,13 @@ module VagrantPlugins
             return value.split("_").first
           end
 
+          # If we can't get the guest additions version by guest property, try
+          # to get it from the VM info itself.
+          info = execute("showvminfo", @uuid, "--machinereadable", :retryable => true)
+          info.split("\n").each do |line|
+            return $1.to_s if line =~ /^GuestAdditionsVersion="(.+?)"$/
+          end
+
           return nil
         end