x86/vm_event: add short-circuit for breakpoints (aka "fast single step")

x86/vm_event: add short-circuit for breakpoints (aka "fast single step")

When using DRAKVUF (or another system using altp2m with shadow pages similar
to what is described in
https://xenproject.org/2016/04/13/stealthy-monitoring-with-xen-altp2m),
after a breakpoint is hit the system switches to the default
unrestricted altp2m view with singlestep enabled. When the singlestep
traps to Xen another vm_event is sent to the monitor agent, which then
normally disables singlestepping and switches the altp2m view back to
the restricted view.

This patch short-circuiting that last part so that it doesn't need to send the
vm_event out for the singlestep event and should switch back to the restricted
view in Xen automatically.

This optimization gains about 35% speed-up.

Was tested on Debian branch of Xen 4.12. See at:
https://github.com/skvl/xen/tree/debian/knorrie/4.12/fast-singlestep

Rebased on master:
https://github.com/skvl/xen/tree/fast-singlestep

Signed-off-by: Sergey Kovalev <valor@list.ru>
Acked-by: Jan Beulich <jbeulich@suse.com>
Acked-by: Tamas K Lengyel <tamas@tklengyel.com>