build: Don't try vsock when sockaddr_vm is missing

On an Alpine 3.15.6 container, AF_VSOCK was defined but none of the
system headers defined sockaddr_vm, making it unusable.  Detect this
at configure time, and require both the struct and the family
definition before allowing vsock connections.  While at it, tweak
README to mention how to run rather than skip the vsock unit tests.