x86/pvh: clearly specify used parameters in vcpu_guest_context

x86/pvh: clearly specify used parameters in vcpu_guest_context

The aim of this patch is to define a stable way in which PVH is
going to do AP bringup.

Since we are running inside of a HVM container, PVH should only need
to set flags, cr3 and user_regs in order to bring up a vCPU, the rest
can be set once the vCPU is started using the bare metal methods.

Signed-off-by: Roger Pau Monné <roger.pau@citrix.com>
Reviewed-off-by: Jan Beulich <jbeulich@suse.com>
Acked-by: George Dunlap <george.dunlap@eu.citrix.com>