Add support for VSOCK to engine/net.c

Add support for VSOCK to engine/net.c

* configure: add option to enable/disable vsock support

* engines/net.c: add vsock support

The  VSOCK  address family facilitates communication between virtual
machines and the host they are running on.

The addressing is formed by 2 integers: <CID, port>
- CID: Context ID, it is the ID assigned to the VM
  0, 1, 2 CIDs are reserved:
  0 - hypervisor CID (rarely used)
  1 - local communication (loopback)
  2 - host CID (the guest can always reach the host using CID=2)

- port: port number on 32bit to reach a specific process

* examples: add 3 simple job files for vsock (one sender, one receiver
  and one that uses vsock loopback interface similar to
  examples/netio.fio)

* fio.1: add vsock to supported protocols together with the required
  parameters

* HOWTO.rst: add vsock to supported protocols together with the required
  parameters

Signed-off-by: Marco Pinna <marco.pinn95@gmail.com>