lguest: add operations to get/set a register from the Launcher.

We use the ptrace API struct, and we currently don't let them set
anything but the normal registers (we'd have to filter the others).

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>

diff --cc drivers/lguest/core.c
Simple merge

diff --cc drivers/lguest/lg.h
Simple merge

diff --cc drivers/lguest/lguest_user.c
Simple merge

diff --cc drivers/lguest/x86/core.c
Simple merge

diff --cc include/linux/lguest_launcher.h
Simple merge