KVM: fix load_guest_segment_descriptor() to return X86EMUL_*

This patch fixes load_guest_segment_descriptor() to return
X86EMUL_PROPAGATE_FAULT when it tries to access the descriptor
table beyond the limit of it: suggested by Marcelo.

I have checked current callers of this helper function,
  - kvm_load_segment_descriptor()
  - kvm_task_switch()
and confirmed that this patch will change nothing in the
upper layers if we do not change the handling of this
return value from load_guest_segment_descriptor().

Next step: Although fixing the kvm_task_switch() to handle the
propagated faults properly seems difficult, and maybe not worth
it because TSS is not used commonly these days, we can fix
kvm_load_segment_descriptor(). By doing so, the injected #GP
becomes possible to be handled by the guest. The only problem
for this is how to differentiate this fault from the page faults
generated by kvm_read_guest_virt(). We may have to split this
function to achive this goal.

Signed-off-by: Takuya Yoshikawa <yoshikawa.takuya@oss.ntt.co.jp>
Signed-off-by: Marcelo Tosatti <mtosatti@redhat.com>