Move exception raising back into retryable block in VirtualBox driver command execution

By moving the exception throwing back into the retryable block it actually enables the retryable to function properly by retrying, if appropriate, when an certain exception is raised.
Merge pull request #2365 from Tomdarkness/fix-virtualbox-driver-retryable

builder/virtualbox: Move exception raising back into retryable block in VirtualBox driver command execution