pvcalls: reuse buffers and evtchns

Do not free and unmap buffers on socket release, reuse them instead.
Add pages and evtchn to a free list in the frontend, then reuse them
when allocating a new socket. Store page mappings and evtchns in an
hashtable in the backend.

Signed-off-by: Stefano Stabellini <stefano@aporeto.com>