unix: SockaddrVM uses "context ID" terminology instead of "client ID"

"When the client creates a connection, instead of providing an IP address
to choose its server, the client must provide the context ID (CID) of a
virtual machine or host."
Reference:
https://pubs.vmware.com/vsphere-60/topic/com.vmware.ICbase/PDF/ws9_esx60_vmci_sockets.pdf

Change-Id: Ia2dfe015594e0ca43a3c8acec1f43d418f5781af
Reviewed-on: https://go-review.googlesource.com/37864Reviewed-by: Ian Lance Taylor <iant@golang.org>

unix/syscall_linux.go

@@ -556,7 +556,7 @@ func (sa *SockaddrALG) sockaddr() (unsafe.Pointer, _Socklen, error) {
 // bidirectional communication between a hypervisor and its guest virtual
 // machines.
 type SockaddrVM struct {
-	// CID and Port specify a client ID and port address for a VM socket.
+	// CID and Port specify a context ID and port address for a VM socket.
 	// Guests have a unique CID, and hosts may have a well-known CID of:
 	//  - VMADDR_CID_HYPERVISOR: refers to the hypervisor process.
 	//  - VMADDR_CID_HOST: refers to other processes on the host.