MAINTAINERS: add VM SOCKETS (AF_VSOCK) entry

Add a new entry for VM Sockets (AF_VSOCK) that covers vsock core,
tests, and headers. Move some general vsock stuff from virtio-vsock
entry into this new more general vsock entry.

I've been reviewing and contributing for the last few years,
so I'm available to help maintain this code.

Cc: Dexuan Cui <decui@microsoft.com>
Cc: Jorgen Hansen <jhansen@vmware.com>
Cc: Stefan Hajnoczi <stefanha@redhat.com>
Suggested-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Stefano Garzarella <sgarzare@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>