KVM: VMX: Handle #SS faults from real mode

Instructions with address size override prefix opcode 0x67
Cause the #SS fault with 0 error code in VM86 mode.  Forward
them to the emulator.

Signed-Off-By: Nitin A Kamble <nitin.a.kamble@intel.com>
Signed-off-by: Avi Kivity <avi@qumranet.com>