powerpc/kvm/booke: Fix build break due to stack frame size warning

Commit ce11e48b ("KVM: PPC: E500: Add
userspace debug stub support") added "struct thread_struct" to the
stack of kvmppc_vcpu_run().  thread_struct is 1152 bytes on my build,
compared to 48 bytes for the recently-introduced "struct debug_reg".
Use the latter instead.

This fixes the following error:

cc1: warnings being treated as errors
arch/powerpc/kvm/booke.c: In function 'kvmppc_vcpu_run':
arch/powerpc/kvm/booke.c:760:1: error: the frame size of 1424 bytes is larger than 1024 bytes
make[2]: *** [arch/powerpc/kvm/booke.o] Error 1
make[1]: *** [arch/powerpc/kvm] Error 2
make[1]: *** Waiting for unfinished jobs....

Signed-off-by: Scott Wood <scottwood@freescale.com>
Signed-off-by: Alexander Graf <agraf@suse.de>