qemu-sockets: add socket_listen, socket_connect, socket_parse

These are QAPI-friendly versions of the qemu-sockets functions.  They
support IP sockets, Unix sockets, and named file descriptors, using a
QAPI union to dispatch to the correct function.

Reviewed-by: Luiz Capitulino <lcapitulino@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>