sockets: move qapi_copy_SocketAddress into qemu-sockets.c

The qapi_copy_SocketAddress method is going to be useful
in more places than just qemu-char.c, so move it into
the qemu-sockets.c file to allow its reuse.

Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Daniel P. Berrange <berrange@redhat.com>