xen: Add V4V implementation

Setup of v4v domains a domain gets created and cleanup
when a domain die. Wire up the v4v hypercall.

Include v4v internal and public headers.

Signed-off-by: Ross Philipson <ross.philipson@citrix.com>
Add specialized rings for connection and notification

Introduce a new hypercall for connect, and add a way to
distinguish type of rings during registration and unregistration