The SocketAddress struct has an "fd" type, which references the name of a
file descriptor passed over the monitor using the "getfd" command. We
currently blindly assume the FD is a socket, which can lead to hard to
diagnose errors later. This adds an explicit check that the FD is actually
a socket to improve the error diagnosis.

Reviewed-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Signed-off-by: Daniel P. Berrange <berrange@redhat.com>