lguest: make write() operation smp aware

This patch makes the write() file operation smp aware. Which means, receiving
the vcpu_id value through the offset parameter, and being well aware to which
vcpu we're talking to.

Signed-off-by: Glauber de Oliveira Costa <gcosta@redhat.com>
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>