docs/misc: add PV Calls Protocol

docs/misc: add PV Calls Protocol

PV Calls is a paravirtualized protocol that allows the implementation of
a set of POSIX functions in a different domain. The PV Calls frontend
sends POSIX function calls to the backend, which implements them and
returns a value to the frontend and acts on the function call.

This version of the document covers networking function calls, such as
connect, accept, bind, release, listen, poll, recvmsg and sendmsg; but
the protocol is meant to be easily extended to cover different sets of
calls. Unimplemented commands return ENOTSUP.

Signed-off-by: Stefano Stabellini <stefano@aporeto.com>
Reviewed-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>