unix: implement Linux AF_VSOCK sockets

Adds the required constants and new SockaddrVM type for working
with Linux VM sockets.  For more information, see:
https://vmsplice.net/~stefan/stefanha-kvm-forum-2015.pdf.

Fixes golang/go#19434

Change-Id: I9aefb35441b2c7dc412bdfb806ebe08d5c20f6b1
Reviewed-on: https://go-review.googlesource.com/37937
Run-TryBot: Matt Layher <mdlayher@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

unix/syscall_linux.go

@@ -551,6 +551,28 @@ func (sa *SockaddrALG) sockaddr() (unsafe.Pointer, _Socklen, error) {
 	return unsafe.Pointer(&sa.raw), SizeofSockaddrALG, nil
 }
 
+// SockaddrVM implements the Sockaddr interface for AF_VSOCK type sockets.
+// SockaddrVM provides access to Linux VM sockets: a mechanism that enables
+// 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.
+	// 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.
+	CID  uint32
+	Port uint32
+	raw  RawSockaddrVM
+}
+
+func (sa *SockaddrVM) sockaddr() (unsafe.Pointer, _Socklen, error) {
+	sa.raw.Family = AF_VSOCK
+	sa.raw.Port = sa.Port
+	sa.raw.Cid = sa.CID
+
+	return unsafe.Pointer(&sa.raw), SizeofSockaddrVM, nil
+}
+
 func anyToSockaddr(rsa *RawSockaddrAny) (Sockaddr, error) {
 	switch rsa.Addr.Family {
 	case AF_NETLINK:
@@ -620,6 +642,14 @@ func anyToSockaddr(rsa *RawSockaddrAny) (Sockaddr, error) {
 			sa.Addr[i] = pp.Addr[i]
 		}
 		return sa, nil
+
+	case AF_VSOCK:
+		pp := (*RawSockaddrVM)(unsafe.Pointer(rsa))
+		sa := &SockaddrVM{
+			CID:  pp.Cid,
+			Port: pp.Port,
+		}
+		return sa, nil
 	}
 	return nil, EAFNOSUPPORT
 }