KVM: PPC: Book3S HV: Don't do ultravisor calls on systems without ultravisor

Commit 22945688 ("KVM: PPC: Book3S HV: Support reset of secure
guest") added a call to uv_svm_terminate, which is an ultravisor
call, without any check that the guest is a secure guest or even that
the system has an ultravisor.  On a system without an ultravisor,
the ultracall will degenerate to a hypercall, but since we are not
in KVM guest context, the hypercall will get treated as a system
call, which could have random effects depending on what happens to
be in r0, and could also corrupt the current task's kernel stack.
Hence this adds a test for the guest being a secure guest before
doing uv_svm_terminate().

Fixes: 22945688

 ("KVM: PPC: Book3S HV: Support reset of secure guest")
Signed-off-by: Paul Mackerras <paulus@ozlabs.org>