Add vm pre-resume hook.

No hooks are currently triggered on resume - including resume after
a live migrate, which is the only possible time a hook could trigger
on a new VM on the target host.
Remove Re_str usage

It's not threadsafe

Signed-off-by: Jon Ludlam <jonathan.ludlam@citrix.com>
Merge pull request #71 from jonludlam/re_str_removal

Remove Re_str usage
Merge pull request #62 from BobBall/resume_hook

Add vm pre-resume hook.