lguest: improve code readability in lg_cpu_start.

Make the container_of call friendlier and fix some comment slip-ups.

Signed-off-by: Cosmin Paraschiv <csmnprschv@gmail.com>
Cc: Daniel Baluta <dbaluta@ixiacom.com>
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>