kvm: ppc: fixes for KVM_SET_SREGS on init

Classic/server ppc has had SREGS for a while now (though I think not
always?), but it's still missing for booke.  Check the capability before
calling KVM_SET_SREGS.

Without this, booke kvm fails to boot as of commit
84b4915dd2c0eaa86c970ffc42a68ea8ba9e48b5 (kvm: Handle kvm_init_vcpu
errors).

Also, don't write random stack state into the non-PVR sregs fields --
have kvm fill it in first.

Eventually booke will have sregs and it will have its own capability to
be tested here.  However, we will want a way for platform code to request
to look like the actual CPU we're running on, especially if SoC devices
are being directly assigned.

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