KVM: x86: zero IDT limit on entry to SMM

The recent BlackHat 2015 presentation "The Memory Sinkhole"
mentions that the IDT limit is zeroed on entry to SMM.

This is not documented, and must have changed some time after 2010
(see http://www.ssi.gouv.fr/uploads/IMG/pdf/IT_Defense_2010_final.pdf

).
KVM was not doing it, but the fix is easy.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>