KVM: x86 emulator: Eliminate compilation warning in x86_decode_insn()

Eliminate:
arch/x86/kvm/emulate.c:801: warning: ‘sv’ may be used uninitialized in this
function

on gcc 4.1.2

Signed-off-by: Sheng Yang <sheng@linux.intel.com>
Signed-off-by: Marcelo Tosatti <mtosatti@redhat.com>