Wait for packets to be accepted before returning from writev

Without this, the caller has no way to know when it's safe to reuse the
buffer. This also works around this problem:

http://lists.xenproject.org/archives/html/xen-users/2014-07/msg00067.html
Merge pull request #11 from talex5/wait-until-accepted

Wait for packets to be accepted before returning from writev