sockets: Support multipath TCP

sockets: Support multipath TCP

Multipath TCP allows combining multiple interfaces/routes into a single
socket, with very little work for the user/admin.

It's enabled by 'mptcp' on most socket addresses:

   ./qemu-system-x86_64 -nographic -incoming tcp:0:4444,mptcp

Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
Message-Id: <20210421112834.107651-6-dgilbert@redhat.com>
Acked-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>