KVM: x86 emulator: add CBW/CWDE/CDQE instruction emulation

Add CBW/CWDE/CDQE instruction emulation.(opcode 0x98)
Used by FreeBSD's boot loader.

Signed-off-by: Wei Yongjun <yjwei@cn.fujitsu.com>
Signed-off-by: Avi Kivity <avi@redhat.com>