KVM: x86: fix mov immediate emulation for 64-bit operands

MOV immediate instruction (opcodes 0xB8-0xBF) may take 64-bit operand.
The previous emulation implementation assumes the operand is no longer than 32.
Adding OpImm64 for this matter.

Fixes https://bugzilla.redhat.com/show_bug.cgi?id=881579

Signed-off-by: Nadav Amit <nadav.amit@gmail.com>
Signed-off-by: Marcelo Tosatti <mtosatti@redhat.com>