sim: dv-sockser: add a write buffer variant

sim: dv-sockser: add a write buffer variant

Rather than having to bang out chunks of data one byte at a time over
the socket interface, add a write variant that accepts an arbitrarily
long buffer.  This speeds things up considerably when we have many
chars to send out at once.

Signed-off-by: Mike Frysinger <vapier@gentoo.org>