VSOCK: extract connect/accept functions from vsock_diag_test.c

Many test cases will need to connect to the server or accept incoming
connections.  This patch extracts these operations into utility
functions that can be reused.

Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: Stefano Garzarella <sgarzare@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>