XenVif #15 & XenNet #7

XenVif #15

Changes
Fix formatting
Re-work checksum code slightly to match reference code
Add some more assertions concerning checksums and large packets.
Two small refinements:
Use split RX/TX event channels if the backend supports them.

XenNet #7

Fix crash in co-installer
Update to latest VIF interface
Merge pull request #27 from benchalmers/xv15-xn7

XenVif #15 & XenNet #7