sockets: add AF_VSOCK support

Add the AF_VSOCK address family so that qemu-ga will be able to use
virtio-vsock.

The AF_VSOCK address family uses <cid, port> address tuples.  The cid is
the unique identifier comparable to an IP address.  AF_VSOCK does not
use name resolution so it's easy to convert between struct sockaddr_vm
and strings.

This patch defines a VsockSocketAddress instead of trying to piggy-back
on InetSocketAddress.  This is cleaner in the long run since it avoids
lots of IPv4 vs IPv6 vs vsock special casing.

Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
* treat trailing commas as garbage when parsing (Eric Blake)
* add configure check instead of checking AF_VSOCK directly
Signed-off-by: Michael Roth <mdroth@linux.vnet.ibm.com>
