lguest: make emulate_insn receive a vcpu struct.

emulate_insn() needs to know about current eip, which will be,
in the future, a per-vcpu thing. So in this patch, the function
prototype is modified to receive a vcpu struct

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